Oriented polymeric articles and method

ABSTRACT

Stretched articles, such as oriented polymeric films or fibers, having microstructure features on at least one surface thereof, and processes for making such articles, are disclosed. A method of making fibers comprises forming a polymeric film having a body having a first surface and a second surface and having a longitudinal dimension. The film also comprises a plurality of elongate microstructure features disposed on the first surface of the body in a direction substantially parallel to the longitudinal dimension of the body, wherein the elongate microstructure features are substantially parallel. The method further comprises stretching the polymeric film in a direction substantially parallel to the longitudinal dimension of the body, and fibrillating the stretched polymeric film along the longitudinal dimension of the body to provide one or more fibers, wherein each fiber has at least one microstructure feature thereon.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims priority to U.S.application Ser. No. 11/427,149, filed Jun. 28, 2006, now allowed.

TECHNICAL FIELD

The present invention relates to stretched articles, such as orientedpolymeric films and fibers, having structured surfaces, and to processesfor making and using such articles.

BACKGROUND

Articles having structured surfaces, and processes for providing sucharticles are known. In the case of optical articles, see, for example,U.S. Pat. Nos. 6,096,247 and 6,808,658, and published application U.S.2002/0154406 A1. The structured surfaces disclosed in these referencesinclude microprisms (such as microcubes) and lenses. Typically thesestructures are created on the surface of a suitable polymer by, forexample, embossing, extrusion or machining.

The manufacture of such articles often comprises a step in which a toolbearing a negative version of the desired structured surface iscontacted with a polymer resin. Contact with the resin is maintained fora time and under conditions adequate to fill the cavities in the toolafter which the resin is removed from the tool. The resulting structuredsurface is a replicate of the negative surface of the tool.

Birefringent articles having structured surfaces are also known. See,for example, U.S. Pat. Nos. 3,213,753; 4,446,305; 4,520,189; 4,521,588;4,525,413; 4,799,131; 5,056,030; 5,175,030 and published applications WO2003/0058383 A1 and WO 2004/062904 A1.

Processes for manufacturing stretched films are also known. Suchprocesses are typically employed to improve the mechanical and physicalproperties of the film. These processes include biaxial stretchingtechniques and uniaxial stretching techniques. See for example PCT WO00/29197, U.S. Pat. Nos. 2,618,012; 2,988,772; 3,502,766; 3,807,004;3,890,421; 4,330,499; 4,434,128; 4,349,500; 4,525,317 and 4,853,602. Seealso U.S. Pat. Nos. 4,862,564; 5,826,314; 5,882,774; 5,962,114 and5,965,247. See also Japanese Unexamined Patent Publications Hei 5-11114;5-288931; 5-288932; 6-27321 and 6-34815. Still other Japanese UnexaminedApplications that disclose processes for stretching films include Hei5-241021; 6-51116; 6-51119; and 5-11113. See also WO 2002/096622 A1.

SUMMARY

One aspect of the invention comprises a method of making fiberscomprising forming a polymeric film having a body having a first surfaceand a second surface and having a longitudinal dimension. The filmcomprises a plurality of elongate microstructure features disposed onthe first surface of the body in a direction substantially parallel tothe longitudinal dimension of the body, wherein the microstructurefeatures are substantially parallel. The method also comprisesstretching the polymeric film in a direction substantially parallel tothe longitudinal dimension of the body, and separating the stretchedpolymeric film along generally longitudinally disposed separation linesto define a plurality of discrete fiber elements, wherein one or more ofthe fiber elements have at least one microstructure feature thereon.

In another aspect, the invention comprises a method of making fiberscomprising forming a polymeric film having a body having a first surfaceand a second surface and having a longitudinal dimension. The film alsocomprises a plurality of elongate microstructure features disposed onthe first surface of the body in a direction substantially parallel tothe longitudinal dimension of the body, wherein the elongatemicrostructure features are substantially parallel. The method furthercomprises stretching the polymeric film in a direction substantiallyparallel to the longitudinal dimension of the body, and fibrillating thestretched polymeric film along the longitudinal dimension of the body toprovide one or more fibers, wherein each fiber has at least onemicrostructure feature thereon.

In one embodiment, the present invention is a polymeric film comprisinga body having a first surface and a second surface, a first thicknessand a longitudinal dimension. The film also comprises a plurality ofelongate microstructure features disposed on the first surface of thebody in a direction substantially parallel to the longitudinal dimensionof the body, wherein the elongate microstructure features aresubstantially parallel and have a second thickness. The film has astretch ratio of at least 1.5 in the longitudinal dimension of the body,and the ratio of the first thickness of the body to the second thicknessof the microstructure features is at most 2.

In yet another embodiment, the present invention is a polymeric fibercomprising a fiber body having a first surface and a second surface, anda longitudinal dimension. The fiber also comprises one or more elongatemicrostructure features disposed on the first surface of the fiber bodyin a direction substantially parallel to the longitudinal dimension ofthe fiber body, wherein any plurality of microstructure features aresubstantially parallel.

This summary is not intended to describe each disclosed embodiment orevery implementation of the present invention. Many other noveladvantages, features, and relationships will become apparent as thisdescription proceeds. The figures and the description that follow moreparticularly exemplify illustrative embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in the followingdetailed description of various embodiments of the invention inconnection with the accompanying drawings, in which:

FIG. 1 is a sectional view of a precursor film useful in the presentinvention.

FIG. 2 is a sectional view of one embodiment film of the presentinvention.

FIGS. 3A-3D are sectional views of some alternative embodiments of thefilm of the present invention.

FIGS. 4A-4D are illustrations useful in determining how to calculate theshape retention parameter (SRP).

FIGS. 5A-5W illustrate sectional views of some alternative profiles ofgeometric features useful in the present invention.

FIG. 6 is a schematic representation of a process according to thepresent invention.

FIG. 7 is a perspective view of a structure surface film both before andafter the stretching process, wherein the film after stretching isuniaxially oriented.

FIG. 8 is a schematic illustration of a method for uniaxially stretchinga film according to the present invention also illustrating a coordinateaxis showing a machine direction (MD), a normal, i.e., thickness,direction (ND), a transverse direction (TD).

FIG. 9 is an end view of an article of the invention having a structuredsurface of varying cross-sectional dimensions.

FIG. 10 is an isometric schematic illustration of a film article of thepresent invention, showing its elongate microstructure features.

FIG. 11 is a sectional view of one embodiment film article of thepresent invention.

FIG. 12 is a sectional view of the film article of FIG. 11, afterseparation in to a plurality of fibers.

FIG. 13 is a sectional view of one embodiment of a fiber article of thepresent invention.

FIG. 14 is an isometric schematic illustration of an array of fiberarticles of the present invention.

FIG. 15 is an isometric exploded schematic illustration of a pluralityof overlying arrays of fiber articles of the present invention.

FIG. 16 is a sectional view of one embodiment of a fiber article of thepresent invention.

FIG. 17 is a schematic representation of a process according to thepresent invention.

FIG. 18 is a representation of a portion of a surface of a micro groovedtool used for forming the film of the present invention as disclosed inExample 8.

FIG. 19 is a photograph of a lateral section taken through a filmarticle of the present invention, as disclosed in Example 8.

FIG. 20 is a schematic representation of a hydroentangler for use infibrillating a film of the present invention.

FIG. 21 is a photograph of a surface of a film of the present inventionin a from according to Example 9.

FIG. 22 illustrates a portion of a surface of a micro grooved tool usedfor forming the film of the present invention as disclosed in Example10.

FIG. 23 is a photograph of a lateral section taken through a filmarticle of the present invention, as disclosed in Example 10.

The invention is amenable to various modifications and alternativeforms. Specifics of the invention are shown in the drawings by way ofexample only. The intention is not to limit the invention to theparticular embodiments described. Instead, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention.

GLOSSARY

As used herein, the following terms and phrases have the followingmeaning.

“Cross sectional shape”, and obvious variations thereof, means theconfiguration of the periphery of the geometric feature defined by thesecond in-plane axis and the third axis. The cross sectional shape ofthe geometric feature is independent of is physical dimension andpresence of defects or irregularities in the feature.

“Stretch ratio”, and obvious variations thereof, means the ratio of thedistance between two points separated along a direction of stretch afterstretching to the distance between the corresponding points prior tostretching.

“Geometric feature”, “feature” and obvious variations thereof, means thepredetermined shape or shapes present on the structured surface.

“Elongate” means extending along a length orientation of a film or fiberbody.

“Micro” is used as a prefix and means that the term that it modifies hasa cross-sectional profile that has a height of 1 mm or less. In oneembodiment, the cross-sectional profile has a height of 0.5 mm or less.In another embodiment, the cross-sectional profile has a height of 0.05mm or less.

“Uniaxial stretch”, including obvious variations thereof, means the actof grasping opposite edges of an article and physically stretching thearticle in only one direction. Uniaxial stretch is intended to includeslight imperfections in uniform stretching of the film due to, forexample, shear effects that can induce momentary or relatively verysmall biaxial stretching in portions of the film.

“Structure surface” means a surface that has at least one geometricfeature thereon.

“Structured surface” means a surface that has been created by anytechnique that imparts a desired geometric feature or plurality ofgeometric features to a surface.

“Electret” means a material that exhibits a quasi-permanent electriccharge.

“Metallic surface” and obvious variations thereof, means a surfacecoated or formed from a metal or a metal alloy which may also contain ametalloid. “Metal” refers to an element such as iron, gold, aluminum,etc., generally characterized by ductility, malleability, luster, andconductivity of heat and electricity which forms a base with thehydroxyl radical and can replace the hydrogen atom of an acid to form asalt. “Metalloid” refers to nonmetallic elements having some of theproperties of a metal and/or forming an alloy with metal (for example,semiconductors) and also includes nonmetallic elements which containmetal and/or metalloid dopants.

“True uniaxial orientation”, and obvious variations thereof, means astate of uniaxial orientation (see below) in which the orientationsensitive properties measured along the second in-plane axis and thethird axis are substantially equal and differ substantially from theorientation sensitive properties along the first in-plane axis.

Real physical systems generally do not have properties which areprecisely and exactly identical along the second in-plane axis and thethird axis. The term “true uniaxial orientation” is used herein to referto a state of orientation in which orientation-sensitive properties ofthe film measured along these axes differ only by a minor amount. Itwill be understood that the permissible amount of variation will varywith the intended application. Often, the uniformity of such films ismore important than the precise degree of uniaxial orientation. Thissituation is sometimes referred to in the art as “fiber symmetry”,because it can result when a long, thin, cylindrical fiber is stretchedalong its fiber axis.

“True uniaxial stretch” and obvious variations thereof, means the act ofproviding uniaxial stretch (see above) in such a manner that the stretchratios along the second in-plane axis and the third axis aresubstantially identical to each other but substantially different fromthe stretch ratio along the first in-plane axis.

“Uniaxial orientation”, including obvious variations thereof, means thatan article has a state of orientation in which orientation sensitiveproperties of the article measured along the first in-plane axis, i.e.,the axis substantially parallel to the uniaxial stretching direction,differ from those measured along the second in-plane axis and the thirdaxis. Though a wide variety of properties may be measured to determinethe presence of uniaxial orientation, crystal orientation and morphologyare the properties of interest herein unless another is specified. Otherillustrative examples of such properties include the refractive index,thermal and hygroscopic expansions, the small strain anisotropicmechanical compliances, tear resistance, creep resistance, shrinkage,the refractive indices and absorption coefficients at variouswavelengths.

In the case of layered films, “uniaxial” or “truly uniaxial” areintended to apply to individual layers of the film unless otherwisespecified.

DETAILED DESCRIPTION

The present invention is directed to methods for structured orientingfilms and fibers, and the articles made thereby. Many of the embodimentsdisclosed herein are directed to the optical characteristics of suchoriented articles and the related processes. However, the invention hassignificant utility beyond optical applications, as is set forth in anumber of alternative embodiments disclosed herein.

The present invention provides a film having a structured surface,articles made therefrom, and a novel process for the manufacturethereof. The structured surface comprises at least one geometric featurehaving a desired cross-sectional shape. One embodiment of the article ofthe invention comprises a film having the structured surface. One aspectof the invention comprises an article that has a uniaxial orientationthroughout its thickness. The structured surface comprises a pluralityof geometric features. The geometric feature or features are elongate.The feature or features are substantially aligned with a first in-planeaxis of the article. The article of the invention comprises a land, orbody, portion having a structured surface thereon. The article maycomprise a single layer or a plurality of separate layers. The articleof the invention may have a structured surface on opposing sidesthereof. The layers may comprise different polymeric materials. Thearticle may be positively or negatively birefringent, electret,crystalline, hydrophilic, hydrophobic, microporous and/or have otherdesired characteristics.

One embodiment of the article of the invention comprises a uniaxiallyoriented structured surface polymeric film comprising:

-   -   (a) a polymeric body having (i) a first and a second surface,        and (ii) first and second in-plane axes that are orthogonal with        respect to each other and a third axis that is mutually        orthogonal to the first and second in-plane axis in a thickness        direction of the polymeric film; and    -   (b) an elongate geometric feature disposed on the first surface        of the polymeric body in a direction substantially parallel to        the first in-plane axis of the polymeric film;        wherein the film has a shape retention parameter (SRP) of at        least 0.1.

The present invention also provides a roll of uniaxially orientedstructured surface article comprising:

-   -   (a) a polymeric body having (i) a first and a second surface,        and (ii) first and second in-plane axes that are orthogonal with        respect to each other and a third axis that is mutually        orthogonal to the first and second in-plane axis in a thickness        direction of the polymeric film; and    -   (b) a surface portion comprising an elongate geometric feature        disposed on the on the first surface of the polymeric body, the        linear geometric feature being disposed on the body in a        direction substantially parallel to the first in-plane axis of        the polymeric film.

In another aspect of the invention, the roll as described abovecomprises a polymeric film that is uniaxially oriented along the firstin-plane axis. In yet another aspect, the roll as described abovefurther comprises a cushioning layer between individual wraps of theroll. The cushioning layer aids in protecting the structured surfacefrom damage and/or distortion during manufacture, storage and shipping.

In yet another aspect of the invention, the article has a firstrefractive index (n₁) along the first in-plane axis, a second refractiveindex (n₂) along the second in-plane axis and a third refractive index(n₃) along the third in-plane axis. In the present invention, n₁ ≠ toeach of n₂ and n₃. That is, n₁ may be greater than n₂ and n₃ or it maybe less than n₂ and n₃. In one embodiment, n₂ and n₃ are substantiallyequal to one another. The relative birefringence of the film of theinvention is, in one embodiment, 0.3 or less.

The present invention may also comprise a multi-phase film. In thisembodiment, the film may comprise a multi-component phase separatingsystem or one in which one component is dissolved in another to createeither a porous structure or very small particles in a continuous matrixor a bi-continuous matrix.

The present invention may also incorporate an additional layer overeither the microstructured surface or the second surface. It may alsoincorporate additional layers on either or both of such surfaces. Theadditional layer can be added before or after stretching. If theadditional layer is added before stretching, it should be capable ofbeing stretched.

Examples of such layers include, but are not limited to, antireflectivelayers, index-matching layers and protective layers.

Truly uniaxial stretching is particularly useful when an additionallayer is employed. In this case, for example, stress build-up in thecross direction is minimized so that factors of adhesion between thelayers is a less critical feature.

In another aspect, the present invention comprises a roll ofmicrostructure film with predetermined properties defined in referenceto a coordinate system of first and second orthogonal in-plane axes anda third mutually orthogonal axis in a thickness direction of the film.For example, the geometric features can be aligned with the direction ofwrap of the roll (i.e., along the machine direction (MD)) or they may bealigned transverse to the direction of wrap of the roll (i.e., along thecross direction (TD)). Alternatively, the geometric structures may bealigned at any desired angle to the MD or TD directions.

The present invention further comprises a method of making a structuredsurface film. One aspect, the method of the invention comprises thesteps of:

-   -   (a) providing a polymeric film having (i) a first surface        comprising a desired geometric feature; and a second surface,        and (ii) first and second in-plane axes that are orthogonal with        respect to each other and a third axis that is mutually        orthogonal to the first and second in-plane axis in a thickness        direction of the polymeric film and subsequently    -   (b) stretching the polymeric film in a direction substantially        parallel to the first in-plane axis of the polymeric film;        wherein the cross sectional shape of the geometric feature        before step (b) is substantially retained after step (b).

In another aspect, the invention comprises a method of making astructured surface film that comprises the steps of:

-   -   (a) providing a polymeric film having (i) a first structured        surface and a second surface, and (ii) first and second in-plane        axes that are orthogonal with respect to each other and a third        axis that is mutually orthogonal to the first and second        in-plane axis in a thickness direction of the polymeric film,        wherein the first structured surface has a geometric feature        disposed thereon in a direction substantially parallel to the        first in-plane axis; and subsequently    -   (b) uniaxially orienting the polymeric film in a direction        substantially parallel to the first in-plane axis of the        polymeric film.

Yet another aspect the invention comprises a method of making astructured surface film that comprises the steps of:

-   -   (a) providing a tool that comprises a negative of a desired        structured surface;    -   (b) contacting the tool with a resin to create the desired        surface, the desired structure surface comprising a geometric        feature;    -   (c) optionally, solidifying the resin to form a film having (i)        the desired structured surface and an opposed surface, and (ii)        first and second in-plane axes that are orthogonal with respect        to each other and a third axis that is mutually orthogonal to        the first and second in-plane axis in a thickness direction of        the film;    -   (d) removing the film from the tool; and subsequently    -   (e) stretching the polymeric film in a direction substantially        parallel to the first in-plane axis of the polymeric film.

Another embodiment of the invention comprises a method of making adesired microstructure surface film having a plurality of elongategeometric micro-features. The method comprising the steps of:

-   -   (a) providing a tool comprising a negative version of the        desired microstructure surface;    -   (b) providing a molten polymeric resin to a gap formed between        the master tool and a second surface;    -   (c) forming a polymeric film having the desired microstructure        surface in the gap, the film having (i) first and second        in-plane axes that are mutually orthogonal with respect to each        other and a third axes that is mutually orthogonal with respect        to the first and second in-plane axes in a thickness direction        of the film, and (ii) the desired microstructure surface having        the elongate micro-features positioned in a direction        substantially parallel to the first in-plane axis;    -   (d) removing the polymeric film of step (c) from the tool; and    -   (e) stretching the polymeric film in a direction substantially        parallel to the first in-plane axis.

In one embodiment of the method(s) of the invention, the article has afirst orientation state prior to stretching and a second orientationstate, different from the first orientation state, after stretching. Inanother embodiment, stretching provides a smaller, physical crosssection (i.e., smaller geometric features).

The method(s) of the invention provide a polymeric film that isstretched, creating, for example, a film that is birefringent afterstretching, and has a first index of refraction (n₁) along the firstin-plane axis, a second index of refraction (n₂) along the secondin-plane axis, and a third index of refraction (n₃) along the thirdaxis.

In another embodiment of the invention, the method creates substantiallythe same proportional dimensional changes in the direction of both ofthe second and third in-plane axes of the film. These proportionaldimensional changes in the direction of the second and third in-planeaxes are substantially the same throughout the stretch or stretchhistory of the film.

In another embodiment, the present invention provides a method by whicha wide variety of polymers can be used to replicate the negative surfaceof a tool. The present invention provides a method of making having apolymeric article having a desired structured surface comprising thesteps of:

-   -   (a) providing a tool that comprises a negative surface of the        desired structured surface;    -   (b) contacting the negative surface of the tool with a        composition comprising a fluorochemical benzotriazole to provide        a coated negative surface;    -   (c) contacting the coated negative surface with a resin to        create the desired structured surface in the resin, the desired        structured surface comprising a geometric feature; and    -   (d) removing the resin from the tool.

The structured surface provided on the article by the process of theinvention comprises a replica of the negative surface of the tool. Thestructured surface of the article has at least one geometric featurehaving a desired cross-sectional shape. One embodiment of the method ofthe invention comprises making a film having the structured surface. Themethod of the invention may be used to make unoriented and orientedarticles such as films. The oriented articles may be uniaxially orbiaxially oriented. The replicated structured surface made by theprocess of the invention may comprise a plurality of geometric features.The geometric feature or features are elongate. The feature or featuresmay be aligned with a first in-plane axis of the article. Alternatively,they may be disposed on the article at any desired angle to the firstin-plane axis. The method may be used to make articles that comprise asingle layer or a plurality of separate layers. The layers may comprisedifferent polymeric materials. The article may be positively ornegatively birefringent, electret, crystalline, hydrophilic,hydrophobic, microporous and/or have other desired characteristics.Additionally, the method of the invention may be used to make articlesthat have a structured surface on both opposing sides thereof.

The geometric feature or features on the article and/or replicated bythe process of the invention are elongate in nature and may be either aprismatic, rectangular, convex, concave, complex or lenticular geometricfeature. The geometric feature or features is a microfeature and may becontinuous or discontinuous along the first in-plane axis. It may have avariety of cross-sectional profiles as discussed more fully below. Thegeometric feature may be repeating or non-repeating on the replicatedsurface. The replicated surface may comprise a plurality of geometricfeatures that have the same cross-sectional shape. Alternatively, it mayhave a plurality of geometric features that have differentcross-sectional shapes.

In another aspect of the invention, the film as manufactured by anymethod of the invention is fibrillated after stretching to provide oneor more uniaxially oriented fibers having a structured surface. Thefibers may be created as individual fibers or as two or more fibersjoined along their length to one another. In one embodiment, one or morefibers may be free of any portion of the structured surface (i.e., haveno geometric features thereon).

The articles and films of the invention generally comprise a bodyportion and a surface structure portion. FIG. 1 represents an end viewof a pre-cursor film having a first orientation state while FIG. 2represents an end view of one embodiment of the film of the inventionhaving a second orientation state FIGS. 3A-3D represent end views ofsome alternative embodiments of the invention.

Precursor film 9 comprises a body or land portion 11 having an initialthickness (Z) and a surface portion 13 having a height (P). Surfaceportions 13 comprises a series of parallel geometric features 15 hereshown as right angle prisms. Geometric features 15 each have a basewidth (BW) and a peak-to-peak spacing (PS). The precursor film has atotal thickness T which is equal to the sum of P+Z.

With specific reference to FIG. 2, the film of the invention 10comprises a body or land portion 12 having a thickness (Z′) and asurface portion 14 having a height (P′). Surface portion 14 comprises aseries of parallel geometric features 16 comprising prisms. Geometricfeatures 16 each have a base width (BW′) and a peak-to-peak spacing(PS′). The film of the invention has a total thickness T′ which is equalto P′+Z′.

The relationship between the dimensions of the precursor film and thefilm of the invention are T′<T; P′<P; Z′<Z; usually BW′<BW; and PS′<PS.

Body or land portions 11, 12 comprise the portion of the article betweenbottom surfaces 17 and 19 and the lowest point of the surface portions15, 16. In some cases, this may be a constant dimension across the width(W,W′) of the article. In other cases, this dimension may vary due tothe presence of geometric features having varying land thicknesses. SeeFIG. 9. In FIG. 9, the land thickness is represented by Z″.

The precursor film 9 and the film of the invention 10 each have a firstin-plane axis 18, a second in-plane axis 20 and a third axis 22 in thethickness direction. The first in-plane axis is substantially parallelto the direction of stretching as discussed herein after. In FIGS. 1 and2, this axis is normal to the end of films 9 and 10. These three axesare mutually orthogonal with respect to one another.

The cross-sectional shape of at least one geometric feature of the filmor article of the present invention substantially mimics thecross-sectional shape of the geometric feature of its precursor. Thisfidelity in shape is especially important when making optical deviceswhere uniform redistribution of incident light is desired. This is truewhether the initial cross-sectional shape of the feature comprises flator curved surfaces. The shape retention of the article and process isdetermined by calculating the Shape Retention Parameter (SRP).

SRP for a given feature is determined as follows. An image is acquiredof a cross-section of a film having the feature before stretching. Thesectioning plane is the plane defined by the second in-plane axis 20 andthe third axis 22 and is orthogonal to the direction in which the filmis to be stretched. One representative example of the structuralfeatures present is chosen, and is referred to as the feature. A line issuperimposed on the image at the junction of the body portion 11 and thesurface portion 13. This is the Feature Baseline (FB). The area of thefeature above its baseline is then calculated. This is the UnstretchedFeature Area (UFA).

An image is then acquired of a cross-section of the film afterstretching. The sectioning plane is the plane defined by the secondin-plane axis and the third axis. If the film has been stretched by anon-continuous, or “batch” process, such as on a laboratory filmstretching instrument, it will be possible to select the same feature asthat selected when examining the film specimen before stretching. If thefilm has been stretched on a continuous film-making line, the featureshould be selected from an appropriate location on the stretched filmweb, analogous to the location that was chosen on the unstretched web,as will be appreciated by one skilled in the film making art. A FeatureBaseline (FB) is again established, and the area of the stretched filmfeature is then calculated. This is the Stretched Feature Area (SFA).

The ratio UFA/SFA is then calculated. This is the Image Ratio (IR). Theimage of the stretched film feature is then scaled up proportionately soas to have the same area as the image of the unstretched film feature.This is done by expanding the image in each of the height and widthdimensions by a factor of the square root of IR. The scaled up image ofthe feature of the stretched film is then superimposed on the image ofthe feature of the unstretched film in such a way that their FeatureBaselines coincide. The superimposed images are then translated withrespect to one another along their common baseline, until the locationis found that maximizes the area of their overlap. This and all theaforementioned and subsequent mathematical and numerical operations canbe done simply on a computer with appropriately written code, as will beapparent to one skilled in the art.

The area shared by both of the superimposed images in this optimallysuperimposed condition is the Common Area (CA). The ratio CA/UFA is thencalculated. This ratio is the Common Area Ratio (CAR). For a stretchthat results in perfect shape retention, the CAR will be unity. For anydeviation from perfect shape retention, the CAR will be a positivenumber less than unity.

For any particular film, CAR will differ from unity by an amount thatdepends at least on the shape of the feature, the stretch ratio, and thedegree to which the stretching operation approaches a truly uniaxiallyorienting stretch. Other factors may also be involved. In order toquantify the degree of deviation from perfect shape retention, it isnecessary to develop another parameter, the Shape Retention Parameter(SRP). The SRP is a measure which indicates proportionately where a filmhaving a structured surface falls, on a continuum, from perfect shaperetention at one extreme, to a selected reference point characteristicof typical industrial practice, at the other extreme. We have chosen assuch a reference point the performance, for the same feature shape andstretch ratio, of an idealized film tenter (transverse orienter)operated efficiently in a continuous mode. The major axis of thefeatures on the film's structured surface is assumed to be parallel tothe crossweb direction, which is the stretch direction. Edge effects andall other process non-idealities are neglected, as are non-idealities ofthe film material itself, such as changes in density upon stretching,for example. For this ideal tenter case, then, all the transversestretch imparted to the film is accommodated by shrinkage of the film,by the same ratio, and in the thickness dimension only. Because thehypothetical tenter is ideal, there is no shrinkage of the film in themachine or downweb direction.

Image Ratio, for a film that stretches ideally, is the same as thestretch ratio. If the Image Ratio is different from the stretch ratio,this is indicative of non-idealities in the system due to, for example,Poisson's Ratio, density changes (e.g., due to crystallization duringstretch) and variations between the local stretch ratio and the nominalideal stretch ratio.

The following will be described with reference to FIGS. 4A-4D. Thecalculations may easily be performed by computer using algorithms knownto those skilled in the art. The calculation begins with theexperimentally obtained image of the feature of the unstretched filmwhich was used already to calculate the CAR. In FIG. 4A, the featureshown is a right triangle feature. The right triangle is shown in FIG.4A only for illustrative purposes as the methodology detailed here isgenerally applicable to any feature shape, whether having or not havingsymmetry, and whether having straight (prismatic) or curved (lenticular)surfaces. The methodology is also generally applicable to “dished”features, or features having complex shapes, such as S-shaped features,hook-shaped features, or “mushroom-cap” features.

The image of FIG. 4A is computationally converted to the image of FIG.4B by shrinking only the height dimension by a factor of the stretchratio used in making the film in question. This simulates what wouldhave happened to the film surface feature in the “ideal tenter” for thefeature shape and stretch ratio in question. The image is then convertedfrom that of FIG. 4B to that of FIG. 4C by scaling it up in each of theheight and width dimensions by a factor of the square root of thestretch ratio. Thus, the image of FIG. 4C has an area identical to thatof the image of FIG. 4A. The images of FIG. 4A and FIG. 4C are thensuperimposed and translated along their common baseline until theposition of maximum overlap area is found. This is shown in FIG. 4D. Thecommon area of this FIG. (the crosshatched area which is common to boththe original feature image and the computationally processed featureimage) is calculated, and the ratio of this area to the area of theimage of FIG. 4A is calculated. This value is the Common Area Ratio forthe Ideal Tenter (CARIT), for the given feature shape and stretch ratio.It will be understood that this calculation must be done independentlyfor each film specimen, as the CARIT is a strong function of both theunstretched feature shape and the stretch ratio employed.

Finally, SRP is calculated using the following formula:SRP=(CAR−CARIT)/(1−CARIT)For perfect shape retention, SRP is unity. For the case of ahypothetical film stretched on an “ideal” tenter, CAR equals CARIT, andSRP is zero. Thus, SRP is a measure which indicates proportionatelywhere a film having a structured surface falls, on a continuum, fromperfect shape retention at one extreme, to a selected reference pointcharacteristic of typical industrial practice, at the other extreme.Films having SRP very close to 1.00 show a very high degree of shaperetention. Films having SRP very close to 0.00 show a low degree ofshape retention for the feature shape and stretch ratio employed. Insome embodiments of the present invention, the films have an SRP of atleast 0.1.

It will be understood by one skilled in the art that a film made on astandard film tenter or by other means may well have an SRP value whichis less than zero, due to the many non-idealities which are possible, asdiscussed above. The “ideal tenter” is not meant to represent the worstpossible shape retention which can result. Rather, it is a point ofreference useful for comparing different films on a common scale.

In one embodiment of the present invention, a film having a structuredsurface has a value of SRP of about 0.1 to 1.00. In another embodimentof the present invention, a film having a structured surface has a valueof SRP of about 0.5 to 1.00. In another embodiment of the presentinvention, a film having a structured surface has a value of SRP ofabout 0.7 to 1.00. In another embodiment of the present invention, afilm having a structured surface has a value of SRP of about 0.9 to1.00.

The method of the invention can be used to make a film possessing auniaxial orientation. The uniaxial orientation may be measured bydetermining the difference in the index of refraction of the film alongthe first in-plane axis (n₁), the index of refraction along the secondin-plane axis (n₂), and the index of refraction along the third axis(n₃). Uniaxially oriented films of the invention have n₁≠n₂ and n₁≠n₃.In one embodiment, the films of the invention are truly uniaxiallyoriented. That is, n₂ and n₃ are substantially equal to one another andrelative to their differences with n₁.

In one embodiment, the method of the invention may be used to provide afilm possessing a relative birefringence of 0.3 or less. In anotherembodiment, the relative birefringence is less than 0.2 and in yetanother embodiment it is less than 0.1. Relative birefringence is anabsolute value determined according to the following formula:|n ₂ −n ₃ |/|n ₁−(n ₂ +n ₃)/2|

Relative birefringence may be measured in either the visible or the nearinfra-red spectra. For any given measurement, the same wavelength shouldbe used. A relative birefringence of 0.3 in any portion of eitherspectra is satisfactory to meet this test.

The method of the invention can be used to make films that comprise atleast one prismatic or lenticular geometric feature. The geometricfeature may be an elongate structure that is generally parallel to thefirst in-plane axis of the film. As shown in FIG. 2, the structuredsurface comprises a series of prisms 16. However, other geometricfeatures and combinations thereof may be used. For example, FIG. 3Ashows that the geometric features do not have to have apices nor do theyneed to touch each other at their bases. As shown in FIG. 3A, when afeature does not have an apice, it may have a feature top width (TW′).When the features do not touch at their bases, adjacent features arespaced apart laterally by a feature separation (FS′) or separation spanof the body (see FIG. 3A).

FIG. 3B shows that the geometric features may have rounded peaks andcurved facets. FIG. 3C shows that the peaks of the geometric featuresmay be flat.

FIG. 3D shows that both opposing surfaces of the film may have astructured surface.

FIGS. 5A-5W illustrate other cross-section shapes that may be used toprovide the structured surface. These Figures further illustrate thatthe geometric feature may comprise a depression (See FIGS. 5A-5I and 5T)or a projection (see FIGS. 5J-5S and 5U-5W). In the case of featuresthat comprise depressions, the elevated area between depressions may beconsidered to be a projection-type feature as shown in FIG. 3C.

Various feature embodiments may be combined in any manner so as toachieve a desired result. For example horizontal surfaces may separatefeatures that have radiused or flat peaks. Moreover curved faces may beused on any of these features.

As can be seen from the Figures, the features may have any desiredgeometric shape, although in some embodiments, they are elongate. One orboth surfaces may also include an element that is not elongate. They maybe symmetric or asymmetric with respect to the z-axis of the film.Moreover, the structured surface may comprise a single feature, aplurality of the same feature in a desired pattern, or a combination oftwo or more features arranged in a desired pattern. Additionally, thedimensions, such as height and/or width, of the features may be the sameacross the structured surface. Alternatively, they may vary from featureto feature.

The microstructure geometric features illustrated in FIG. 2 eithercomprise or approximate a right angle prism. As used herein, a rightangle prism has an apex angle of from about 70° to about 120°, morelikely from about 80° to 100°, most likely about 90°. While a rightangle prism is illustrated, prisms of other angular configurations arealso contemplated. Additionally the faces of the microstructure featureare flat or approximate a flat surface.

In another embodiment, the microstructure geometric features comprise asaw tooth-like prism. As used herein a saw tooth-like prism has avertical, or nearly vertical side that forms an approximately 90° anglewith the land or body. See FIG. 5J. In one useful embodiment, asaw-tooth-like prism may have has an angle of inclination from the landor body of from 2° to 15°.

It is also within the scope of the present invention that the featuresmay be either continuous or discontinuous along the first in-plane axis.

Various embodiments of the film of the invention comprise the followingdimensional relationships as set forth in FIGS. 2 and 3A:

A process of the invention generally comprises the steps of providing astructured surface polymeric film that is capable of being elongated bystretching and subsequently uniaxially stretching the film. Thestructured surface may either be provided concurrently with theformation of the film or it may be imparted to the first surface afterthe film has been formed. The process will be further explained withregard to FIGS. 6 and 7.

FIG. 6 is a schematic representation of a method according to thepresent invention. In the method, a tool 24 comprising a negativeversion of the desired structured surface of the film is provided and isadvanced by means of drive rolls 26A and 26B past an orifice (not shown)of die 28. Die 28 comprises the discharge point of a melt train, herecomprising an extruder 30 having a feed hopper 32 for receiving drypolymeric resin in the form of pellets, powder, etc. Molten resin exitsdie 28 onto tool 24. A gap 33 is provided between die 28 and tool 24.The molten resin contacts the tool 24 and hardens to form a polymericfilm 34. The leading edge of the film 24 is then stripped from the tool24 at stripper roll 36 and is directed to uniaxial stretching apparatus38. The stretched film may then be wound into a continuous roll atstation 40.

It should be noted that film 34 may be wound into a roll, or cut intosheets and stacked before being stretched in apparatus 38. It shouldalso be noted that film 34 may be cut into sheets after being stretchedrather than being wound into a continuous roll.

The film 34 may optionally be pre-conditioned (not shown) before theuniaxial stretching. Additionally, the film 34 may be post-conditioned(not shown) after stretching.

A variety of techniques may be used to impart a structured surface tothe film. These include batch and continuous techniques. They mayinvolve providing a tool having a surface that is a negative of thedesired structured surface; contacting at least one surface of thepolymeric film to the tool for a time and under conditions sufficient tocreate a positive version of the desired structured surface to thepolymeric film; and removing the polymeric film with the structuredsurface from the tool. Typically the negative surface of the toolcomprises a metallic surface.

Although the die 28 and tool 24 are depicted in a vertical arrangementwith respect to one another, horizontal or other arrangements may alsobe employed. Regardless of the particular arrangement, the die 28provides the molten resin to the tool 24 at the gap 33.

The die 28 is mounted in a manner that permits it to be moved toward thetool 24. This allows one to adjust the gap 33 to a desired spacing. Thesize of the gap 33 is a factor of the composition of the molten resin,the desired body thickness, its viscosity, its viscoelastic responses,and the pressure necessary to essentially completely fill the tool withthe molten resin as will be understood by those in the art.

The molten resin is of a viscosity such that it substantially fills,optionally with applied vacuum, pressure, temperature, ultrasonicvibration or mechanical means, into the cavities of the tool 24. Whenthe resin substantially fills the cavities of the tool 24, the resultingstructured surface of the film is said to be replicated.

The negative surface of the tool can be positioned to create featuresacross the width of the film (i.e., in the transverse (TD) direction) oralong the length of the film (i.e., along the machine (MD) direction).Perfect alignment with the TD or MD direction is not required. Thus thetool may be slightly off angle from perfect alignment. Typically, thismisalignment is no more than about 20°.

In the case that the resin is a thermoplastic resin, it is typicallysupplied as a solid to the feed hopper 32. Sufficient energy is providedto the extruder 30 to convert the solid resin to a molten mass. The toolis typically heated by passing it over a heated drive roll 26A. Driveroll 26A may be heated by, for example circulating hot oil through it orby inductively heating it. For some of the optical applicationsdisclosed herein, the temperature of the tool 24 is typically from 20°C. below the softening point of the resin to the decompositiontemperature of the resin. For other applications, the temperature of thetool is held below the softening point of the resin due to low viscosityand less stringent structure needs.

In the case of a polymerizable resin, including a partially polymerizedresin, the resin may be poured or pumped directly into a dispenser thatfeeds the die 28. If the resin is a reactive resin, the method of theinvention may include one or more additional steps of curing the resin.For example, the resin may be cured by exposure to a suitable radiantenergy source such as actinic radiation such as ultraviolet light,infrared radiation, electron beam radiation, visible light, etc., for atime sufficient to harden the resin and remove it from the tool 24.

The molten film can be cooled by a variety of methods to harden the filmfor further processing. These methods include spraying water onto theextruded resin, contacting the unstructured surface of the tool withcooling rolls, or direct impingement of the film with air.

The previous discussion was focused on the simultaneous formation of thefilm and the structured surface. Another technique useful in theinvention comprises contacting a tool to the first surface of apreformed film. Pressure, heat or pressure and heat are then applied tothe film/tool combination until the surface of the film has softenedsufficiently to create the desired structured surface in the film. Usingthis technique, the surface of the film is softened sufficiently tosubstantially completely fill the cavities in the tool. Subsequently,the film is cooled and removed from the tool.

In yet another technique, a preformed film may be machined, such as bydiamond turning, to create a desired structured surface thereon.

As noted previously, the tool comprises a negative version (i.e., thenegative surface) of the desired structured surface. Thus, it comprisesprojections and depressions (or cavities) in a predetermined pattern.The negative surface of the tool can be contacted with the resin so asto create the geometric features on the structured surface in anyalignment with respect to the first or second in-plane axes. Thus, forexample, the geometric features of FIG. 1 may be aligned with either themachine, or length, direction, or the transverse, or width, direction ofthe article.

It is typical that a release agent be applied to the tool to enhanceremoval of the resin from the tool. For example, organic materials suchas oils and waxes and silicones have been used as release agents toprovide release characteristics to surfaces. One of the disadvantages ofthese release agents is that they usually need to be frequentlyre-applied to the surface so as to provide adequate release properties.Polymeric release coatings such as those made frompolytetrafluoroethylenes have addressed some of the shortcomings ofoils, waxes, silicones and other temporary coatings and are often moredurable. Typically however, polymeric release coatings require a thickercoating than the non-durable treatments, they can be subject tothickness variations, and can present application difficulties.

Additionally, it has been found that certain classes of polymers do notseparate reliably and cleanly from the tool. Consequently, it isdifficult to replicate the negative surface of the tool with suchpolymers.

In one embodiment of the replication step, the cavities of the tool areat least 50% filled by the resin. In another embodiment, the cavitiesare at least 75% filled by the resin. In yet another embodiment, thecavities are at least 90 percent filled by the resin. In still anotherembodiment, the cavities are at least 95% filled by the resin. In eventanother embodiment, the cavities are at least 98% filled by the resin.

Adequate fidelity to the negative may be achieved for many applicationswhen the cavities are filled to at least 50% by the resin. However,better fidelity to the negative is achieved when the cavities are filledto at least 90% by the resin. The best fidelity to the negative isachieved when the cavities are filled to at least 98% by the resin.

The tool used to create the desired structured surface has a coatingcomprising a fluorochemical benzotriazole on the negative surface. Inone embodiment, the fluorochemical benzotriazoles forms a substantiallycontinuous monolayer film on the tool. The molecules form “substantiallycontinuous monolayer film” means that the individual molecules packtogether as densely as their molecular structures allow. It is believedthat the films self assemble in that the triazole groups of themolecules of the invention attach to available areas of themetal/metalloid surface of the tool and that the pendant fluorocarbontails are aligned substantially towards the external interface.

The effectiveness of a monolayer film and the degree to which amonolayer film is formed on a surface is generally dependent upon thestrength of the bond between the compound and the particular metal ormetalloid surface of the tool and the conditions under which thefilm-coated surface is used. For example, some metal or metalloidsurfaces may require a highly tenacious monolayer film while other suchsurfaces require monolayer films having much lower bond strength. Usefulmetal and metalloid surfaces include any surface that will form a bondwith compounds of the invention and, in one embodiment, form a monolayeror a substantially continuous monolayer film. Examples of suitablesurfaces for forming said monolayer films include those comprisingcopper, nickel, chromium, zinc, silver, germanium, and alloys thereof.

The monolayer or substantially continuous monolayer film may be formedby contacting a surface with an amount of the fluorochemicalbenzotriazole sufficient to coat the entire surface. The compound may bedissolved in an appropriate solvent, the composition applied to thesurface, and allowed to dry. Suitable solvents include ethyl acetate,2-propanol, acetate, 2 propanol, acetone, water and mixtures thereof.Alternatively, the fluorochemical benzotriazole may be deposited onto asurface from the vapor phase. Any excess compound may be removed byrinsing the substrate with solvent and/or through use of the treatedsubstrate.

The fluorochemical benzotriazoles not only have been found to chemicallybond to metal and metalloid surfaces, they also provide, for example,release and/or corrosion inhibiting characteristics to those surfaces.These compounds are characterized as having a head group that can bondto a metallic or metalloid surface (such as a master tool) and a tailportion that is suitably different in polarity and/or functionality froma material to be released. These compounds form durable, self-assembledfilms that are monolayers or substantially monolayers. Thefluorochemical benzotriazoles include those having the formula:

wherein R_(f) is C_(n) F_(2n+1) —(CH₂)_(m)—, wherein n is an integerfrom 1 to 22 and m is 0, or an integer from 1 to 6; X is —CO₂—, —SO₃—,—CONH—, —O—, —S—, a covalent bond, —SO₂NR—, or —NR—, wherein R is H orC₁ to C₅ alkylene; Y is —CH₂— wherein z is 0 or 1; and R′ is H, loweralkyl or R_(f)—X—Y_(z)— with the provisos that when X is —S—, or —O—, mis 0, and z is 0, n is ≧7 and when X is a covalent bond, m or z is atleast 1. In one embodiment, n+m is equal to an integer from 8 to 20.

A particularly useful class of fluorochemical benzotriazole compositionsfor use as release agents comprising one or more compounds having theformula:

wherein R_(f) is C_(n) F_(2n+1) —(CH²)_(m)—, wherein n is 1 to 22, m is0 or an integer from 1 to 6; X is —CO₂—, —SO₃—, —S—, —O—, —CONH—, acovalent bond, —SO₂NR—, or —NR—, wherein R is H or C₁ to C₅ alkylene,and q is 0 or 1; Y is C₁-C₄ alkylene, and z is 0 or 1; and R′ is H,lower alkyl, or R_(f)—X—Y_(z). Such materials are described, forexample, in U.S. Pat. No. 6,376,065.

The process may optionally include a preconditioning step prior tostretching such as providing an oven or other apparatus. Thepreconditioning step may include a preheating zone and a heat soak zone.The stretch ratios may also be reduced from its maximum to controlshrinkage. This is known in the art as “toe in”.

The process may also include a post conditioning step. For example, thefilm may be first heat set and subsequently quenched.

Uniaxial stretching can occur in a conventional tenter or in a lengthorienter. A general discussion of film processing techniques can befound in “Film Processing”, edited by Toshitaka Kanai and GregoryCampbell, 1999, Chapters 1, 2, 3, and 6. See also “The Science andTechnology of Polymer Films,” edited by Orville J. Sweeting, 1968, Vol.1, pages 365-391 and 471-429. Uniaxial stretching can also be achievedin a variety of batch devices such as between the jaws of a tensiletester.

Uniaxial stretching processes include, but are not limited to,conventional “length orientation” between rollers rotating at differentspeeds, conventional cross-web stretching in a tenter, stretching in aparabolic-path tenter such as that disclosed in WO WO2002/096622 A1, andstretching between the jaws of a tensile tester.

For an ideal elastic material, uniaxial orientation will result if twoof three mutually orthogonal stretch ratios are identical. For amaterial which undergoes no significant change in density uponstretching, each of the two substantially identical stretch ratios willbe substantially equal to the square root of the reciprocal of the thirdorthogonal stretch ratio.

Films stretched in a conventional tenter, although uniaxially oriented,are not truly uniaxially oriented even though they have been uniaxiallystretched, because the film is not free to contract along the axis ofthe direction of travel through the tenter, but is free to contract inthe thickness direction. Films stretched in parabolic-path tenters, suchas those disclosed in WO2002/096622 A1, are both uniaxially stretchedand truly uniaxially oriented, because the parabolic path allows for anappropriate amount of contraction of the film along the axis of travelthrough the tenter. Processes other than parabolic-path tentering mayalso provide true uniaxial orientation, and the concept is not meant tobe limited by the process employed.

True uniaxial orientation is also not limited to those processes thatstretch film under uniaxial conditions throughout the entire history ofthe stretch. In one embodiment, deviation from a uniaxial stretch ismaintained within certain tolerances throughout the various portions ofthe stretching step. However, processes in which deviations fromuniaxiality early in a stretching process are compensated for later inthe stretching process, and which yield true uniaxiality in theresulting film are also included in the scope of the invention.

Herein, the path traveled by the gripping means of the tenter stretchingapparatus which grips a film edge, and hence, the path traced by an edgeof the film as it travels through the tenter, is referred to as aboundary trajectory. It is within the present invention to provide aboundary trajectory that is three dimensional and substantiallynon-planar. The film may be stretched out-of-plane using out-of-planeboundary trajectories, that is, boundary trajectories that do not lie ina single Euclidean plane.

Though it is not required for true uniaxiality, using the parabolic-pathtenter process, the film is stretched in-plane. In one embodiment,straight lines stretched along TD, the principal stretch direction,remain substantially straight after stretching. In conventional tenterprocessing of films, this is typically not the case, and lines sostretched acquire a substantial curvature or “bow”.

The boundary trajectories may be, but do not need to be, symmetrical,forming mirror images through a central plane. This central plane is aplane passing through a vector in the initial direction of film traveland passing through the initial center point between the boundarytrajectories, and a vector normal to the surface of the unstretched filmbeing fed to the stretching apparatus.

Like other film stretching processes, parabolic-path tentering benefitsfrom the selection of conditions such that a uniform spatial drawing ofthe film is maintained throughout the stretching process. Good spatialuniformity of the film may be achieved for many polymeric systems withcareful control of the crossweb and downweb thickness distribution ofthe unstretched film or web and careful control of the temperaturedistribution across the web throughout the stretch. Many polymericsystems are particularly sensitive to non-uniformities and will stretchin a non-uniform fashion if caliper and temperature uniformity areinadequate. For example, polypropylenes tend to “line stretch” underuniaxial stretching. Certain polyesters, notably polyethylenenaphthalate, are also very sensitive.

Whichever stretching technique is employed, stretching should be donesubstantially parallel to the first in-plane axis when shape retentionof the geometric features is desired. It has been found that the moreparallel the stretching is to the first in-plane axis, the better theshape retention that is achieved. Good shape retention can be achievedwhen the deviation from exactly parallel is no more than 20°. Bettershape retention is achieved if the deviation is no more than 10° fromexactly parallel. Even better shape retention is achieved if thedeviation is no more than 5° from parallel.

The parabolic stretching step also can maintain the deviation from auniaxial stretch within certain tolerances throughout the variousportions of the stretching step. Additionally, these conditions can bemaintained while deforming a portion of the film out-of-plane in aninitial portion of the stretch, but return the film in-plane during afinal portion of the stretch.

In a truly uniaxial transverse stretch maintained throughout the entirehistory of the stretch, the instantaneous machine direction stretchratio (MDDR) approximately equals the square root of the reciprocal ofthe transverse direction stretch ratio (TDDR) as corrected for densitychanges. As discussed above, the film may be stretched out-of-planeusing out-of-plane boundary trajectories, i.e. boundary trajectoriesthat do not lie in a single Euclidean plane. There are innumerable, butnevertheless particular, boundary trajectories meeting relationalrequirements of this embodiment of the present invention, so that asubstantially uniaxial stretch history may be maintained usingout-of-plane boundary trajectories.

Following stretching, the film may be heat set and quenched if desired.

Referring now to FIG. 7, an unstretched structured surface film 34 hasdimensions T, W and L, respectively representing the thickness, width,and length of the film. After the film 34 is stretched by a factor oflambda (λ), the stretched film 35 has the dimensions T′, W′, and L′respectively representing the stretched thickness, stretched width, andthe stretched length of the film. This stretching imparts uniaxialcharacter to the stretched film 35.

The relationship between the stretch ratios along the first in-planeaxis, the second in-plane axis and the third axis is an indication ofthe fiber symmetry, and hence the uniaxial orientation of the stretchedfilm. In the present invention, the film has a minimum stretch ratioalong the first in-plane axis of at least 1.1. In one embodiment, thestretch ratio along the first in-plane axis is at least 1.5. In anotherembodiment of the invention, the stretch ratio is at least 1.7. In yetanother embodiment, it is at least 3. Higher stretch ratios are alsouseful. For example, a stretch ratio of 3 to 10 or more is useful in theinvention.

The stretch ratios along the second in-plane axis and the third axis aretypically substantially the same in the present invention. Thissubstantial sameness is most conveniently expressed as the relativeratio of these stretch ratios to one another. If the two stretch ratiosare not equal, then the relative ratio is the ratio of the largerstretch ratio along one of these axes to the smaller stretch ratio alongthe other of the axes. In one embodiment, the relative ratio is lessthan 1.4. When the two ratios are equal the relative ratio is 1.

In the case of truly uniaxial stretching with a stretch ratio of X alongthe first in-plane direction, when the process creates substantially thesame proportional dimensional changes in the second in-plane axis and inthe thickness direction of the film along the third axis, the thicknessand the width will have been reduced by the same proportionaldimensional changes. In the present case, this may be approximatelyrepresented by KT/λ^(0.5) and KW/λ^(0.5) where K represents a scalefactor that accounts for density changes during stretch. In the idealcase, K is 1. When the density decreases during stretching, K is greaterthan 1. When density increases during stretching, K is less than 1.

In the invention, the ratio of the final thickness T′ to initialthickness of the film T may be defined as the NDSR stretch ratio (NDSR).The MDSR may be defined as the length of a portion of the film afterstretching divided by the initial length of that portion. Forillustrative purposes only, see Y′/Y in FIG. 8. The TDSR may be definedas the width of a portion of the film after stretching divided by theinitial width of that portion. For illustrative purposes only, see X′/Xin FIG. 8.

The first in-plane direction may coincide with the MD, e.g., in the caseof a length orientation, or TD, e.g., in the case of a parabolic tenter.In another example, sheets rather than a continuous web are fed into atenter in the so-called batch tentering process. This process isdescribed in U.S. Pat. No. 6,609,795. In this case the first in-planedirection or axis coincides with TD.

The present invention is applicable generally to a number of differentstructured surface films, materials and processes where a uniaxialcharacteristic is desired. The process of the present invention isbelieved to be particularly suited to fabrication of polymeric filmshaving a microstructured surface where the visco-elastic characteristicsof materials used in the film are exploited to control the amount, ifany, of molecular orientation induced in the materials when the film isstretched during processing. The improvements include one or more ofimproved optical performance, enhanced dimensional stability, increasedtensile properties, electete properties, better processability and thelike.

In general, polymers used in the present invention may be crystalline,semi-crystalline, liquid crystalline or amorphous polymers orcopolymers. It should be understood that in the polymer art it isgenerally recognized that polymers are typically not entirelycrystalline, and therefore in the context of the present invention,crystalline or semi-crystalline polymers refer to those polymers thatare not amorphous and includes any of those materials commonly referredto as crystalline, partially crystalline, semi-crystalline, etc. Liquidcrystalline polymers, sometimes also referred to as rigid-rod polymers,are understood in the art to possess some form of long-range orderingwhich differs from three-dimensional crystalline order.

The present invention contemplates that any polymer eithermelt-processable or curable into film form may be used. These mayinclude, but are not limited to, homopolymers, copolymers, and oligomersthat can be further processed into polymers from the following families:polyesters (e.g., polyalkylene terephthalates (e.g., polyethyleneterephthalate, polybutylene terephthalate, andpoly-1,4-cyclohexanedimethylene terephthalate), polyethylene bibenzoate,polyalkylene naphthalates (e.g. polyethylene naphthalate (PEN) andisomers thereof (e.g., 2,6-, 1,4-, 1,5-, 2,7-, and 2,3-PEN)) andpolybutylene naphthalate (PBN) and isomers thereof), and liquidcrystalline polyesters); polyarylates; polycarbonates (e.g., thepolycarbonate of bisphenol A); polyamides (e.g. polyamide 6, polyamide11, polyamide 12, polyamide 46, polyamide 66, polyamide 69, polyamide610, and polyamide 612, aromatic polyamides and polyphthalamides);polyether-amides; polyamide-imides; polyimides (e.g., thermoplasticpolyimides and polyacrylic imides); polyetherimides; polyolefins orpolyalkylene polymers (e.g., polyethylenes, polypropylenes,polybutylenes, polyisobutylene, and poly(4-methyl)pentene); ionomerssuch as Surlyn™ (available from E. I. du Pont de Nemours & Co.,Wilmington, Del.); polyvinylacetate; polyvinyl alcohol andethylene-vinyl alcohol copolymers; polymethacrylates (e.g., polyisobutylmethacrylate, polypropylmethacrylate, polyethylmethacrylate, andpolymethylmethacrylate); polyacrylates (e.g., polymethyl acrylate,polyethyl acrylate, and polybutyl acrylate); polyacrylonitrile;fluoropolymers (e.g., perfluoroalkoxy resins, polytetrafluoroethylene,polytrifluoroethylene, fluorinated ethylene-propylene copolymers,polyvinylidene fluoride, polyvinyl fluoride,polychlorotrifluoroethylene, polyethylene-co-trifluoroethylene,poly(ethylene-alt-chlorotrifluoroethylene), and THV™ (3M Co.));chlorinated polymers (e.g., polyvinylidene chloride andpolyvinylchloride); polyarylether ketones (e.g., polyetheretherketone(“PEEK”)); aliphatic polyketones (e.g., the copolymers and terpolymersof ethylene and/or propylene with carbon dioxide); polystyrenes of anytacticity (e.g., atactic polystyrene, isotactic polystyrene andsyndiotactic polystyrene) and ring- or chain-substituted polystyrenes ofany tacticity (e.g., syndiotactic poly-alpha-methyl styrene, andsyndiotactic polydichlorostyrene); copolymers and blends of any of thesestyrenics (e.g., styrene-butadiene copolymers, styrene-acrylonitrilecopolymers, and acrylonitrile-butadiene-styrene terpolymers); vinylnaphthalenes; polyethers (e.g., polyphenylene oxide,poly(dimethylphenylene oxide), polyethylene oxide and polyoxymethylene);cellulosics (e.g., ethyl cellulose, cellulose acetate, cellulosepropionate, cellulose acetate butyrate, and cellulose nitrate);sulfur-containing polymers (e.g., polyphenylene sulfide, polysulfones,polyarylsulfones, and polyethersulfones); silicone resins; epoxy resins;elastomers (e.g., polybutadiene, polyisoprene, and neoprene), nylons andpolyurethanes. Blends or alloys of two or more polymers or copolymersmay also be used.

As noted above, it has been difficult to replicate surfaces using somepolymers, especially polyesters. Generally they adhere tenaciously tothe tool during the replication process. As a result, they are difficultto remove from the tool without causing damage to the replicatedsurface. Examples of semicrystalline thermoplastic polymers useful inthe invention include semicrystalline polyesters. These materialsinclude polyethylene terephthalate or polyethylene naphthalate. Polymerscomprising polyethylene terephthalate or polyethylene naphthalate arefound to have many desirable properties in the present invention.

Suitable monomers and comonomers for use in polyesters may be of thediol or dicarboxylic acid or ester type. Dicarboxylic acid comonomersinclude but are not limited to terephthalic acid, isophthalic acid,phthalic acid, all isomeric naphthalenedicarboxylic acids (2,6-, 1,2-,1,3-, 1,4-, 1,5-, 1,6-, 1,7-, 1,8-, 2,3-, 2,4-, 2,5-, 2,8-), bibenzoicacids such as 4,4′-biphenyl dicarboxylic acid and its isomers,trans-4,4′-stilbene dicarboxylic acid and its isomers, 4,4′-diphenylether dicarboxylic acid and its isomers, 4,4′-diphenylsulfonedicarboxylic acid and its isomers, 4,4′-benzophenone dicarboxylic acidand its isomers, halogenated aromatic dicarboxylic acids such as2-chloroterephthalic acid and 2,5-dichloroterephthalic acid, othersubstituted aromatic dicarboxylic acids such as tertiary butylisophthalic acid and sodium sulfonated isophthalic acid, cycloalkanedicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid and itsisomers and 2,6-decahydronaphthalene dicarboxylic acid and its isomers,bi- or multi-cyclic dicarboxylic acids (such as the various isomericnorbornane and norbornene dicarboxylic acids, adamantane dicarboxylicacids, and bicyclo-octane dicarboxylic acids), alkane dicarboxylic acids(such as sebacic acid, adipic acid, oxalic acid, malonic acid, succinicacid, glutaric acid, azelaic acid, and dodecane dicarboxylic acid), andany of the isomeric dicarboxylic acids of the fused-ring aromatichydrocarbons (such as indene, anthracene, pheneanthrene, benzonaphthene,fluorene and the like). Other aliphatic, aromatic, cycloalkane orcycloalkene dicarboxylic acids may be used. Alternatively, esters of anyof these dicarboxylic acid monomers, such as dimethyl terephthalate, maybe used in place of or in combination with the dicarboxylic acidsthemselves.

Suitable diol comonomers include but are not limited to linear orbranched alkane diols or glycols (such as ethylene glycol, propanediolssuch as trimethylene glycol, butanediols such as tetramethylene glycol,pentanediols such as neopentyl glycol, hexanediols,2,2,4-trimethyl-1,3-pentanediol and higher diols), ether glycols (suchas diethylene glycol, triethylene glycol, and polyethylene glycol),chain-ester diols such as3-hydroxy-2,2-dimethylpropyl-3-hydroxy-2,2-dimethylpropyl-3-hydroxy-2,2-dimethylpropanoate, cycloalkane glycols such as 1,4-cyclohexanedimethanol andits isomers and 1,4-cyclohexanediol and its isomers, bi- or multicyclicdiols (such as the various isomeric tricyclodecane dimethanols,norbornane dimethanols, norbornene dimethanols, and bicyclo-octanedimethanols), aromatic glycols (such as 1,4-benzenedimethanol and itsisomers, 1,4-benzenediol and its isomers, bisphenols such as bisphenolA, 2,2′-dihydroxy biphenyl and its isomers, 4,4′-dihydroxymethylbiphenyl and its isomers, and 1,3-bis(2-hydroxyethoxy) benzene and itsisomers), and lower alkyl ethers or diethers of these diols, such asdimethyl or diethyl diols. Other aliphatic, aromatic, cycloalkyl andcycloalkenyl diols may be used.

Tri- or polyfunctional comonomers, which can serve to impart a branchedstructure to the polyester molecules, can also be used. They may be ofeither the carboxylic acid, ester, hydroxy or ether types. Examplesinclude, but are not limited to, trimellitic acid and its esters,trimethylol propane, and pentaerythritol.

Also suitable as comonomers are monomers of mixed functionality,including hydroxycarboxylic acids such as parahydroxybenzoic acid and6-hydroxy-2-naphthalenecarboxylic acid, and their isomers, and tri- orpolyfunctional comonomers of mixed functionality such as5-hydroxyisophthalic acid and the like.

Suitable polyester copolymers include copolymers of PEN (e.g.,copolymers of 2,6-, 1,4-, 1,5-, 2,7-, and/or 2,3-naphthalenedicarboxylic acid, or esters thereof, with (a) terephthalic acid, oresters thereof; (b) isophthalic acid, or esters thereof; (c) phthalicacid, or esters thereof; (d) alkane glycols; (e) cycloalkane glycols(e.g., cyclohexane dimethanol diol); (f) alkane dicarboxylic acids;and/or (g) cycloalkane dicarboxylic acids (e.g., cyclohexanedicarboxylic acid)), and copolymers of polyalkylene terephthalates(copolymers of terephthalic acid, or esters thereof, with (a)naphthalene dicarboxylic acid, or esters thereof; (b) isophthalic acid,or esters thereof; (c) phthalic acid, or esters thereof; (d) alkaneglycols; (e) cycloalkane glycols (e.g., cyclohexane dimethane diol); (f)alkane dicarboxylic acids; and/or (g) cycloalkane dicarboxylic acids(e.g., cyclohexane dicarboxylic acid)). The copolyesters described mayalso be a blend of pellets where at least one component is a polymerbased on one polyester and other component or components are otherpolyesters or polycarbonates, either homopolymers or copolymers.

The film of the invention may also contain a disperse phase comprisingpolymeric particles or oil or other incompatible phase separatingmaterial in a continuous polymeric matrix or a bi-continuous matrix ofphases. In an alternative, embodiment of the invention, the dispersephase may be present in one or more of the layers of a multilayer film.The level of polymeric particles used is not critical to the presentinvention and is selected so as to achieve the purposes for which thefinal article is intended. Factors which may affect the level and typeof the polymer particles include the aspect ratio of the particles, thedimensional alignment of the particles in the matrix, the volumefraction of the particles, the thickness of the structured surface film,etc. Typically, the polymer particles are chosen from the same polymersdescribed above.

Films made in accordance with the present invention may be useful for awide variety of products including tire cordage, reinforcement,filtration media, tape backings, wipes such as skin wipes, medicaldressings, bandages, microfluidic films, membranes, blur filters,polarizers, reflective polarizers, dichroic polarizers, alignedreflective/dichroic polarizers, absorbing polarizers, retarders(including z-axis retarders), diffraction gratings, polarizing beamsplitters, brightness enhancement films and polarizing diffractiongratings. The films may comprise the particular element itself or theycan be used as a component in another element such as a tire, industrialbelting, hoses, a dressing, a bandage, a face mask, a respirator, afilter, an adhesive tape, a wipe, a membrane, beamsplitters for frontand rear projection systems, or as a brightness enhancement film used ina display or microdisplay.

The present inventive concept relates to attenuated polymeric structures(film, filament, single or multilayer) and to a method of obtaining suchstructures; and more particularly, to such a method wherein a polymericstructure comprising highly ordered replicated features is stretched toan extent that the features and underlying substrate are deformed in acontrolled and cooperative manner, from the original cast or extrudedform. Polymer (thermoplastic or extensible) structures (solid ormicroporous) of the concept are, in one embodiment, characterized bycontinuous (unbroken in at least one direction along the surface)replicated features (replicated to some geometric constraint betweenfeatures) covering at least a portion of the surface. Structures of theconcept are formed by attenuating a film (cast or profile extruded),with replicated features on at least a portion of the surface, along amajor axis of the structure. Attenuation cooperatively elongates thefeatures and base of the cast or extruded form to produce films orfilaments with replicated surfaces. Attenuated structures of the conceptcan be produced with unique and useful combinations of geometricconformation and underlying structure. As an example, highly molecularlyoriented (and crystallized) films can be produced with fine surfacetopographies that would be useful in applications like rubberreinforcement, filtration, or light management. These materials could befurther processed through a fibrillation step to formmicrostructured-surface fiber structures for filtration. A furtherexample of the unique combination of conformational and structuralproperties would be a microreplicated film made from phase-separablematerials. An attenuated film of this structure would be microporous andcarry a microreplicated surface topography. Materials such as this couldfind application in liquid filtration or water resistant wearingapparel. Attenuated filaments with replicated surface features wouldfind applications in carpeting and personal care application wherecoverage (extended surface area) of the fiber is desirable.

In one embodiment, a polymeric film made according to the presentinvention, as formed prior to stretching, has a body having a firstsurface and a second surface and has one or more elongate microstructurefeatures on at least one surface thereof. Such a film is illustrated,for example, in FIG. 10 as film 109, with a body or land portion 111, afirst surface portion 113, a second surface portion 119, and elongatemicrostructure features 115 disposed on the first surface portion 113.The features are substantially parallel and extend substantiallyparallel to a longitudinal dimension L of the film body, and may becontinuous along a length of the film (e.g., like features 115 in FIG.10) or discontinuous along a length of the film (i.e., not extendingalong the entire length of the film; e.g., like feature 115 b in FIG.10). In lateral cross section, each feature has a pre-stretched shape(e.g., triangular, polygonal, etc.) and first thickness, and the bodyhas a first thickness. Adjacent features may be contiguous when theyjoin the body (see, e.g., FIG. 2), or at least some of the features maybe spaced apart laterally by a separation span of the body (see, e.g.,feature separation FS' in FIG. 3A). In one embodiment, at least one ofthe microstructure features has a discontinuity thereon, such as feature115 a (FIG. 10) bearing discontinuity 141 thereon, or may have aplurality of discontinuities thereon, such as features 115 b and 115 cbearing discontinuities 142 and 143, respectively, thereon. Adiscontinuity on a feature comprises any change in geometry on a surfaceof the feature, including for example, a shallow indentation such asdiscontinuities 141 and 143, or a discontinuity that extends through theentire thickness of a feature to the body, such as discontinuity 142. Adiscontinuity extends longitudinally along a feature, may extendlaterally completely across a feature, or may extend only partially intoa feature, such as a dimple or groove. In addition, a discontinuity mayconstitute a projection from a feature, such as a bump or ridge.

The film is stretched in a direction substantially parallel to thelongitudinal dimension of the body, which elongates the body andmicrostructure features thereon. The body thus assumes a secondthickness, similar or smaller than its first thickness (i.e., Z′<Z). Thefeature thickness may likewise by reduced upon stretching (i.e., P′<P).In one embodiment, the ratio after stretching of body thickness tofeature thickness (i.e., body thickness divided by feature thickness)may range from 2 to 1, in another embodiment may be from 1 to 0.5, andin yet another embodiment the ratio of body thickness to featurethickness may as low as about 0.10. Accordingly, the body thickness maybe relatively thin compared to the thickness of the features, asillustrated in exemplary stretched inventive film of FIG. 19. The bodymay provide just enough material to retain the features bonded togetherin a film structure for further processing or handling. In this way, theelongate microstructure features are joined together by a relativelythin, flexible body or land.

In this form, the stretched film has been oriented throughout itsentirety, including both body and microstructure features. In otherwords, the morphology of each of the microstructure features is the sameas the body-both have been oriented by stretching in the same manner todefine a molecularly oriented polymeric film having a body with at leastone surface thereof bearing elongate microstructure features. In oneembodiment of the present invention, the film has a minimum stretchratio in the longitudinal dimension of at least 1.1. In one embodiment,that stretch ratio is at least 1.5. In another embodiment of theinvention, that stretch ratio is at least 1.7. In yet anotherembodiment, it is at least 3. Higher stretch ratios are also useful. Forexample, a stretch ratio of 3 to 20 or more is useful in the invention.

In one embodiment, the film is crystalline (not amorphous) andhomogeneous. The stretched film exhibits anisotropic physicalproperties, relative to the longitudinal dimension of the body. Forinstance, Young's modulus of the oriented film in the longitudinaldimension is much different (i.e., higher) than Young's modulus in alateral dimension or in a body thickness dimension. Similarly, thetensile strength of the film in the longitudinal dimension is muchdifferent (i.e., higher) than in a lateral dimension or in a bodythickness dimension. Such unique anisotropic properties and morphologyare believed to be achievable in the microstructured oriented film ofthe present invention, but are not achievable by machiningmicrostructure features onto an already stretched film. This is becausethe machining process generates heat that alters the morphology of thefilm material. The oriented crystalline structure of the polymeric filmis altered by the exposure to the thermal effects (i.e., heat) caused bythe friction involved in the film material machining process. Thus, thepresent invention provides a process to create a molecularly orientedpolymeric film having elongate microstructure features on at least oneside thereof in a simple and efficient manner. The film may be cast andthen stretched, rather than cast, stretched and then machined.Accordingly, the elimination of the machining step not only allows theformation of an oriented film having relatively uniform morphologythroughout, it is more economical.

The stretched film has been elongated past its elongation point toorient the crystalline molecules therein. However, if the stretched filmis later heated to a temperature above its glass transition state (butbelow its melting point), the longitudinally oriented crystallinealignment will break down and revert to a more random pattern. This willcause the film to shrink in the longitudinal dimension.

In one form, as mentioned above, the inventive film may be used as areinforcing material for such applications as tire cordage. For vehicletires, cordage material (e.g., like the reinforcing steel belts in“steel belted radials”) is provided to reinforce the tire rubber andprovide added strength and durability to the finished tire. Themolecularly oriented polymeric film of the present invention canlikewise serve this cordage purpose, since its microstructure featuresare oriented, and held in that orientation by the film's body, and thusprovides an array of joined fibers or cords (i.e., the microstructuredfeatures) that are relatively strong in the longitudinal dimension ofthe film's body. The film may be oriented annularly about the axis of atire being formed to provide hoop strength, in one or as many layers asdesired. The layers may be aligned so that the longitudinal dimensionsof the layered films are substantially parallel, or they are different.For instance, in another embodiment, one or more layers of such film maybe aligned in alternative orientations (such as, for example, with thelongitudinal dimension of the film's body parallel to the tire's axis,radiating from the tire's axis, or in an arc about the tire's axis). Inaddition, the film layers may be spaced apart, or they may be engaged,such that at least some of the features or structures on a first layerengage the second surface of the body of a second layer overlying thefirst layer. In addition, the relatively thin body provides a means forhandling the elongate features for precise placement within the tire asit is being molded about the reinforcing cordage film, thus simplifyingthe production process of the reinforced tire. As opposed to a flat filmthat might be used for cordage purposes, the relatively high surfacearea of the inventive film's features and body also allow for enhancedbonding of the rubber to the film during tire assembly. Thischaracteristic is even further enhanced by discontinuities in themicrostructure features, since that creates additional feature surfacearea for bonding. Further, the polymeric material may be lighter inweight than prior cordage materials, while providing equivalent cordstrength.

In another form, as mentioned above, the inventive film may be used as areinforcing material for other applications, such as tape backings. Whendisposed with the longitudinal dimension of the film's body generallyparallel to the length of a tape strip on a backing for the tape (orserving itself as the tape backing), the inventive film provideslongitudinal reinforcement for the tape. In one embodiment, an adhesiveis disposed on at least one surface of the body such as, for example, apressure sensitive adhesive.

In another form, as mentioned above, the inventive film may be formedfrom a multi-phase film, where one component is in another in the filmto create a porous structure. For example, the initial film that is castor molded to have microstructure features on one side of its body may bea high density polyethylene (HDPE) with mineral oil therein. Uponstretching of this film, the spaces in the HDPE that were holding themineral oil are elongated and become voids where the oil was. Inaddition, the stretched film may be further processed to remove much ofthe mineral oil therein (by known techniques). The stretching thus formsoriented microcavities in the longitudinal dimension, and the stretchedfilm becomes a microporous membrane material that may be, for example,vapor permeable but not liquid permeable. The removal of mineral oilmakes the film even more porous. If the film is further stretched in alateral dimension (i.e., is thus biaxially oriented), the cavities orvoids can be even further enlarged in volume and film porosity furtherincreased. The microporous stretched film material (with microstructurefeatures on at least one side thereof) can be used as a singlemicroporous film material, or may be stacked upon similar film materiallayers to achieve a stacked array of microfluidic porous films. Theelongate microstructure features thus serve to define fluidic flowchannels between film layers, by spacing apart the bodies of layers offilms. The stacked film layers may be aligned with the longitudinaldimensions of their bodies generally parallel, or they may be aligned indifferent directions (e.g., perpendicular). As noted above, separationspans may be provided between adjacent elongate microstructure features.Such separation spans may further enhance the fluidic flow capacity ordiversion capabilities of the inventive film when used as a microporousmembrane, by forming larger fluidic flow channels between adjacentfeatures. Discontinuities on the microstructure features, such asdiscontinuities 141, 142 or 143, permit fluidic flow across a featureand thus between adjacent flow channels (such as between channels 144,145, 146 and 147 shown on film 109 in FIG. 10. Providing discontinuitieson the microstructure features may also increase the surface area of thefeatures (and any microporous film or fibers they are disposed on).

In another form, as mentioned above, the inventive film may befibrillated after stretching to provide one or more longitudinallyoriented fibers that may be useful for such applications as theformation of filtration media. To do so, the body connecting theelongate microstructure features is separated along generallylongitudinally disposed separation lines, with each section of theseparated body including one or more microstructure features thereon(although in one embodiment, there may be selected sections of the bodywhich bear no features thereon). While the relatively thin body mayseparate under fibrillation on its own accord, the process of formingthe film may also include the defining of longitudinal separation lineson the film. Those separation lines then define where the film separatesto form fibers during fibrillation. As noted, a fiber may include onlyone microstructure feature, or may include several features (withconnective portions of the body remaining between the several featuresto bind them together). For example, as seen in FIG. 11, a stretchedfilm 110 having a body or land portion 112, a first surface portion 114,a second surface portion 117, and elongate microstructure features 116disposed on the first surface portion 114, may have separation lines 121formed on the body 112 of the film 110. In some cases, adjacent features116 may have a separation span 123 therebetween. The separation linesmay be between adjacent features (such as separation line 121 a) or maybe disposed in a separation span, such as separation line 121 b, andthus spaced laterally from the microstructure features on each side ofthat separation span 121 b. A separation line 121 may be defined in thebody 112 (such as, for example, a score line or weakened segment of thebody) for use in separating the stretched film 110 into fibers or fiberelements 125 (see FIG. 12) during fibrillation. In this case,fibrillation is the process of splitting a longitudinally oriented filminto a network of interconnected fibers, and may be achieved byfibrillation processes known in the art.

Fibrillation of the inventive film results in a plurality of oriented,crystalline and homogenous polymeric fibers having generally uniformmorphology. The features on the fibers may have the same characteristicsas the features on the stretched film from which the fibers are formed.For instance, the features on the fibers have a high feature thicknessto body thickness relationship, which is not possible to attain bymachining of an oriented film to create the features.

As seen, for example, in FIG. 13, each fiber 125 thus formed may have afiber body or land portion 212, a first surface portion 214, a secondsurface portion 217, and one or more elongate microstructure features216 disposed on the first surface portion 214 of the fiber body 212.Each fiber body 212 has a longitudinal dimension (like the film 110 thatformed it), and the features 216 likewise are disposed in a directionsubstantially parallel to the longitudinal dimension of the fiber body212, and are substantially parallel. The fibers can be cut laterally andthen mixed together to form a filtration layer of mixed fibers in anydesired shape (e.g., a filter pad, a filter cylinder, a filter cone,etc.) and density. Alternatively, the fibers may be maintained in anarray where the fiber bodies of the fibers in the array aresubstantially parallel, such as illustrated schematically by array 127of fibers 125 in FIG. 14. In this configuration, the array 127 of fibers125 may also be used for filtration, or in a reinforcement capacity suchas tire cordage or tape backing, as referenced above. Like the film asalso discussed above, an array of fibers may serve as a layer and layersmay over lie each other, such as illustrated schematically in FIG. 15 byoverlaying arrays 127 a, 127 b and 127 c of fibers. Overlaying layers orarrays may be engaged, or may be spaced apart, and the longitudinaldimensions of the fibers in such arrays may be aligned substantiallyparallel or may be disposed in different directions.

Additionally, the fibers may be charged with an electric potential tofurther aid in their filtration capacities. To do so, the polymeric filmis exposed to an electrical field (e.g., by passing the film by a coronawire) to define the film as an electret. The exposing step may precedethe stretching step for the film, or may be done after the film isstretched. On the film, at least one of the microstructure features isshaped to enhance electrical field effects. Thus, when the film isseparated into fibers, each fiber is an electret, and in one embodiment,at least one of the microstructure features on a fiber is shaped toenhance electrical field effects. As illustrated, for example, in FIG.16, a fiber 125 has a fiber body 212 that has a number of edges orpoints 250 (in lateral view) that create electrical potentialdiscontinuities around the fiber body 212. These focused charge areas(i.e., edges 250) on the fiber 125 can further aid the fiber in afiltration context by electrically attracting particulate materials tothe fiber. Hence, the more such edges on a fiber, the better, forfiltration purposes. The inventive film, when segmented into fibers,allows the creation of fibers having microstructure features thereonwith any desired number of edges. In addition, the formation of surfacediscontinuities on a microstructure feature can define additional edgesalong the elongate length of the feature and, in turn, create additionalelectrical potential discontinuities along the fiber body. This not onlycreates more electrical potential field effect attraction for the fiber,but also increases its surface area for filtration purposes. The resultof a filtration layer of such fibers is a highly effective filtrationmedium. As noted above, the stretched film itself may be defined as anelectret by charge application. Thus, when the film is used for fluidicflow purposes or in a filtration context, it also can attract particlesto it to collect those particles.

In one embodiment, when forming a polymeric film of the presentinvention (for use either as a film or to form fibers from a film), atleast one generally laterally disposed separation line may be definedacross the longitudinal dimension of the body of the film. For example,such a lateral separation line is illustrated by line 255 across thefilm 109 of FIG. 10. Such a separation line 255 may be defined as, forexample, a score line or weakened segment of the body and/ormicrostructure features along the line 255 for use in separating thefilm or fiber (after stretching) into fibers element segments. Forfibers, such segment separation may constitute a separate processingstep or may occur at the same time as fibrillation. Alternatively, afilm (e.g., tape) may be formed with a plurality of lateral separationlines along its longitudinal dimension but is not separated into tapesegments until such time as the use defines a desired length of tape.The lateral separation line then facilitates the tearing off of aselected tape segment.

In addition to the other characteristics noted herein, the inventivefilm (and the fibers formed therefrom) may be formed as a microporousfilm (or fiber) that contains microparticlate material. Themicroparticulate material (which can be one material or a combination ofmaterials) useful in the present invention is non-swellable in aqueousand organic media and is substantially insoluble in water or the elutionsolvent. Not more than 1.0 gram of particulate will dissolve in 100 g.of aqueous media or elution solvent into which particulate is mixed at20.degree C. The particulate material can be an organic compound, apolymer, or an inorganic oxide such as silica, alumina, titania,zirconia, and other ceramics, or it can be ion exchange or chelatingparticles. Suitable particles for the purposes of this invention includeany particle which can be coated with insoluble, non-swellable sorbentmaterial or the surface (external and/or internal) of which can bederivatized to provide a coating of insoluble, non-swellable sorbentmaterial thereon. Such particles in film are disclosed in U.S. Pat. No.4,810,381.

In addition, other additives (e.g., a catalyst (e.g., CO oxidativecatalytically active nano-gold particles), carbon fibers or particles,or pigment materials) may be added to the film to provide it or thefibers created thereby with desired properties. In addition, the filmand fibers formed therefrom may be adsorbent or absorbent.

In the above description, the position of elements has sometimes beendescribed in terms of “first”, “second”, “third”, “top” and “bottom”.These terms have been used merely to simplify the description of thevarious elements of the invention, such as those illustrated in thedrawings. They should not be understood to place any limitations on theuseful orientation of the elements of the present invention.

Accordingly, the present invention should not be considered limited tothe particular examples described above, but rather should be understoodto cover all aspects of the invention as fairly set out in the claims.Various modifications, equivalents, as well as numerous structures towhich the present invention may be applicable will be readily apparentto those of skill in the art to which the present invention is directedupon review of the present specification. The claims are intended tocover such modifications and devices.

EXAMPLES Example 1

A polyethylene terephthalate (PET) with an inherent viscosity (I.V.) of0.74 available from Eastman Chemical Company, Kingsport, Tenn. was usedin this example.

The PET pellets were dried to remove residual water and loaded into theextrusion of an extruder hopper under a nitrogen purge. The PET wasextruded with a increasing temperature profile of 232° C. to 282° C.within the extruder and the continuing melt train through to the die setat 282° C. Melt train pressures were continuously monitored and anaverage taken at the final monitored position along the melt train priorto bringing the die into close proximity to the tool onto which thepolymer film is formed simultaneously with the structuring of a firstsurface of that film against the tool.

The tool was a structured belt having a negative version of thestructured surface formed on the cast film. The structured surfacecomprised a repeating and continuous series of triangular prisms. Thetriangles formed a sawtooth-like pattern. The basal vertices of theindividual prisms were shared by their adjoining, neighboringstructures. The prisms were aligned along the casting or machinedirection (MD) direction. The structured surface of the tool was coatedwith a fluorochemical benotriazole having the formula

where R_(f) is C₈F₁₇ and R is —(CH₂)₂—, as disclosed in U.S. Pat. No.6,376,065. The tool was mounted on a temperature-controlled rotating canwhich provides a continuous motion of the tool surface along the casting(MD) direction. The measured surface temperature of the tool averaged92° C.

The die orifice through which the molten polymer exits the melt trainwas brought into close proximity with the rotating belt tool forming afinal slot between the tool and die. The pressure at the final monitoredposition along the melt train increased as the die and tool becamecloser. The difference between this final pressure and the previouslyrecorded pressure is referred to as the slot pressure drop. The slotpressure drop in this example was 7.37×10⁶ Pa (1070 psi) providingsufficient pressure to drive the molten polymer into the structuredcavities formed by the tool negative. The film thereby formed andstructured, was conveyed by the tool rotation from the slot, quenchedwith additional air cooling, stripped from the tool and wound into aroll. Including the height of the structures, the total thickness of thecast film (T) was about 510 microns.

The cast and wound polymer film closely replicated the tool structure.Using a microscope to view the cross-section a prismatic structure wasidentified on the surface of the film with an approximately 85° apexangle, 20° inclination from the horizontal of the film land for one legof the triangle and a 15° tilt from the perpendicular for the oppositeleg. The measured profile exhibited the expected, nearly righttriangular form with straight edges and a slightly rounded apex. Thereplicated prisms on the polymeric film surface were measured to have abasal width (BW) of 44 microns and a height (P) of 19 microns. Thepeak-to-peak spacing (PS) was approximately the same as the basal width(BW). The tool is also imperfect and small deviations from nominalsizing can exist.

The structured cast film was cut into sheets with an aspect ratio of10:7 (along the grooves:perpendicular to grooves), preheated to about100° C. as measured in the plenums and stretched to a nominal stretchratio of 6.4 and immediately relaxed to a stretch ratio of 6.3 in anearly truly uniaxial manner along the continuous length direction ofthe prisms using a batch tenter process. That is individual sheets werefed to a conventional continuous operation film tenter. The relaxationfrom 6.4 to 6.3 is accomplished at the stretch temperature to controlshrinkage in the final film. The structured surfaces maintained aprismatic shape with reasonably straight cross-sectional edges(reasonably flat facets) and approximately similar shape. The basalwidth after stretch (BW′) was measured by microscopy cross-sectioning tobe 16.5 microns and the peak height after stretch (P′) was measured tobe 5.0 microns. The final thickness of the film (T′), including thestructured height, was measured to be 180 microns. The indices ofrefraction were measured on the backside of the stretched film using aMetricon Prism Coupler as available from Metricon, Piscataway, N.J., ata wavelength of 632.8 nm. The indices along the first in-plane (alongthe prisms), second in-plane (across the prisms) and in the thicknessdirection were measured to be 1.672, 1.549 and 1.547 respectively. Therelative birefringence in the cross-sectional plane of this stretchedmaterial was thus 0.016.

Example 2

A polyethylene terephthalate (PET) with an inherent viscosity (I.V.) of0.74 available from Eastman Chemical Company, Kingsport, Tenn. was usedin this example.

The PET pellets were dried to remove residual water and loaded into theextrusion hopper under a nitrogen purge. The PET was extruded with aflat temperature profile about 282° C. within the extruder and thecontinuing melt train through to the die set at 282° C. Melt trainpressures were continuously monitored and an average taken at the finalmonitored position along the melt train prior to bringing the die intoclose proximity to the tool onto which the polymer film is formedsimultaneously with the structuring of a first surface of that filmagainst the tool.

The tool was a structured belt having the desired negative version ofthe structured surface formed on the cast film. The structured surfacecomprised a repeating and continuous series of isosceles righttriangular prisms, with basal widths (BW) of 50 microns and height (P)of nearly 25 microns. The basal vertices of the individual prisms wereshared by their adjoining, neighboring structures. The prisms werealigned along the casting (MD) direction.

The structured surface of the tool was coated with a fluorochemicalbenezotriazole having the formula

where R_(f) is C₄F₉ and R is —(CH₂)₆—. The tool was mounted on atemperature-controlled rotating can which provides a continuous motionof the tool surface along the casting (MD) direction. The measuredsurface temperature of the tool averaged 98° C.

The die orifice through which the molten polymer exits the melt trainwas brought into close proximity with the rotating belt tool forming afinal slot between the tool and die. The pressure at the final monitoredposition along the melt train increased as the die and tool becamecloser. The difference between this final pressure and the previouslyrecorded pressure is referred to as the slot pressure drop. The slotpressure drop in this example was 7.92×10⁶ Pa (1150 psi) providingsufficient pressure to drive the molten polymer into the structuredcavities formed by the tool negative. The film thereby formed andstructured, was conveyed by the tool rotation from the slot, quenchedwith additional air cooling, stripped from the tool and wound into aroll. Including the height of the structures, the total thickness of thecast film (T) was about 600 microns.

The cast and wound polymer film closely replicated the tool structure.Using contact profilometry, (e.g. a KLA-Tencor P-10 with a 60° 2 micronradius stylus). a clear, reasonably sharp prismatic structure wasidentified on the surface of the film. The measured profile exhibitedthe expected, nearly right triangular form with straight edges and aslightly rounded apex. The replicated prisms on the polymeric filmsurface were measured to have a basal width (BW) of 50 microns and aheight (P) of 23.4 microns. The peak-to-peak spacing (PS) wasapproximately the same as the basal width (BW). The profilometry islimited to about a micron in resolution due to the shape and size of thestylus probe and the actual apex may be considerably higher. The tool isalso imperfect and small deviations from nominal sizing can exist. Aratio of the profile-measured cross-sectional area to the idealcalculated cross-sectional area provided a calculated fill of 99%.

The structured film can be stretched in a manner similar to that inExample 1.

Example 3

A polyethylene naphthalate (PEN) with an inherent viscosity (I.V.) of0.56 was made in a reactor vessel.

The PEN pellets were dried to remove residual water and loaded into theextrusion hopper under a nitrogen purge. The PEN was extruded with aflat temperature profile of 288° C. within the extruder and thecontinuing melt train through to the die set at 288° C. Melt trainpressures were continuously monitored and an average taken at the finalmonitored position along the melt train prior to bringing the die intoclose proximity to the tool onto which the polymer film is formedsimultaneously with the structuring of a first surface of that filmagainst the tool.

The tool was a structured belt having the desired negative version ofthe structured surface formed on the cast film. The structured surfacecomprised a repeating and continuous series of isosceles righttriangular prisms, with basal widths (BW) of 50 microns and height (P)of nearly 25 microns. The basal vertices of the individual prisms wereshared by their adjoining, neighboring structures. The prisms werealigned along the casting (MD) direction. The structured surface of thetool was coated with a fluorochemical benzotriazole having the formula

where R_(f) is C₈F₁₇ and R is —(CH₂)₂—. The tool was mounted on atemperature-controlled rotating can which provides a continuous motionof the tool surface along the casting (MD) direction. The measuredsurface temperature of the tool averaged 144° C.

The die orifice through which the molten polymer exits the melt trainwas brought into close proximity with the rotating belt tool forming afinal slot between the tool and die. The pressure at the final monitoredposition along the melt train increased as the die and tool becamecloser. The difference between this final pressure and the previouslyrecorded pressure is referred to as the slot pressure drop. The slotpressure drop in this example was 5.51×10⁶ Pa (800 psi) providingsufficient pressure to drive the molten polymer into the structuredcavities formed by the tool negative. The film thereby formed andstructured, was conveyed by the tool rotation from the slot, quenchedwith additional air cooling, stripped from the tool and wound into aroll. Including the height of the structures, the total thickness of thecast film (T) was about 600 microns.

The cast and wound polymer film closely replicated the tool structure.Using contact profilometry, (e.g. a KLA-Tencor P-10 with a 60° 2 micronradius stylus). A clear, reasonably sharp prismatic structure wasidentified on the surface of the film. The measured profile exhibitedthe expected, nearly right triangular form with straight edges and aslightly rounded apex. The replicated prisms on the polymeric filmsurface were measured to have a basal width (BW) of 50 microns and aheight (P) of 23.3 microns. The peak-to-peak spacing (PS) wasapproximately the same as the basal width (BW). The profilometry islimited to about a micron in resolution due to the shape and size of thestylus probe and the actual apex may be considerably higher. The tool isalso imperfect and small deviations from nominal sizing can exist. Tobetter characterize the actual extent of fill, e.g. characterize theprecision of replication with the tool, the profilometry cross-sectionwas fit to a triangle. Using data from the measured profile, the edgeswere fit as straight lines along the legs of the cross-section between 5and 15 micron height as measured from the base. An ideal apex height of24.6 microns was calculated. A ratio of the profile-measuredcross-sectional area to the ideal calculated cross-sectional areaprovided a calculated fill of 98.0%.

The structured cast film was stretched in a nearly truly uniaxial manneralong the continuous length direction of the prisms, using a batchtenter process. The film was preheated to nominally 165° C. as measuredin the plenums and stretched at this temperature over 25 seconds at auniform speed (edge separation) to a final stretch ratio of about 6. Thestructured surfaces maintained a prismatic shape with reasonablystraight cross-sectional edges (reasonably flat facets) andapproximately similar shape.

Table 1 shows the effect of stretching at various distances from thecenter of the cast film.

Ratio of In-plane In-plane Relative Nominal higher to refractiverefractive Refractive Distance Length Thick. lower cross Thickness PeakHeight Peak width index index index Relative from Stretch Stretchsectional (T′) (P′) (BW′) along perp. through Bire- Center Ratio Ratiostretch ratios microns Microns Microns stretch to stretch thicknessfringence 0.000 0.427 0.381 1.12 230 8.4127 22.025 1.8095 1.5869 1.57850.0370 0.044 0.427 0.385 1.11 230 8.4494 21.95385 1.81 1.5873 1.57810.0405 0.089 0.427 0.377 1.13 230 8.4226 22.08462 1.8101 1.5869 1.57790.0395 0.133 0.427 0.414 1.03 250 8.3739 22.16154 1.8101 1.5871 1.57780.0409 0.178 0.427 0.385 1.11 230 8.3923 22.05 1.8104 1.5866 1.57810.0373 0.222 0.422 0.377 1.12 230 8.3194 21.9286 1.8132 1.5859 1.57990.0261 0.267 0.417 0.368 1.13 220 8.1205 21.85 1.8153 1.5859 1.57780.0347 0.311 0.417 0.352 1.18 210 7.8141 21.9143 1.8166 1.5859 1.57520.0453 0.356 0.411 0.335 1.23 200 7.4737 21.9615 1.818 1.5875 1.57440.0553 0.400 0.406 0.322 1.26 190 7.1668 22.1071 1.8173 1.5887 1.5720.0705 0.444 0.406 0.31  1.31 190 6.8934 22.5143 1.8166 1.5908 1.57270.0771 0.489 0.411 0.301 1.37 180 6.6182 22.85 1.8161 1.5917 1.57180.0849 0.533 0.417 0.289 1.44 170 6.3933 23.4154 1.8146 1.5924 1.57140.0902 0.578 0.422 0.272 1.55 160 5.8504 24.2167 1.8163 1.5979 1.56860.1257 0.622 0.438 0.264 1.66 160 5.6835 25.3154 1.8131 1.5988 1.56620.1414 0.667 0.458 0.264 1.73 160 5.6538 26.8769 1.8112 1.6014 1.56430.1625 0.711 0.484 0.26  1.86 160 5.6149 28.725 1.8111 1.6112 1.56150.2211 0.756 0.51  0.251 2.03 150 5.5633 30.8818 1.811 1.6089 1.55790.2241 0.800 0.552 0.247 2.23 150 5.4791 33.77 1.8117 1.6128 1.5520.2652 0.844 0.594 0.243 2.44 150 5.6443 36.075 1.8143 1.6164 1.54540.3042 Relative distance from center = distance from center/one half ofthe width of the film

Example 4

A polyethylene naphthalate (PEN) with an inherent viscosity (I.V.) of0.56 was made in a reactor vessel.

The PEN pellets were dried to remove residual water and loaded into theextrusion hopper under a nitrogen purge. The PEN was extruded with aflat temperature profile of 288° C. within the extruder and thecontinuing melt train through to the die set at 288° C. Melt trainpressures were continuously monitored and an average taken at the finalmonitored position along the melt train prior to bringing the die intoclose proximity to the tool onto which the polymer film is formedsimultaneously with the structuring of a first surface of that filmagainst the tool.

The tool was a structured belt having the desired negative version ofthe structured surface formed on the cast film. The structured surfacecomprised a repeating and continuous series of isosceles righttriangular prisms, with basal widths (BW) of 50 microns and height (P)of nearly 25 microns. The basal vertices of the individual prisms wereshared by their adjoining, neighboring structures. The prisms werealigned along the casting (MD) direction. The structured surface of thetool was coated with a fluorochemical benzotriazole having the formula

where R_(f) is C₈F₁₇ and R is —(CH₂)₂—, as disclosed in U.S. Pat. No.6,376,065. The tool was mounted on a temperature-controlled rotating canwhich provides a continuous motion of the tool surface along the casting(MD) direction. The measured surface temperature of the tool averaged153° C.

The die orifice through which the molten polymer exits the melt trainwas brought into close proximity with the rotating belt tool forming afinal slot between the tool and die. The pressure at the final monitoredposition along the melt train increased as the die and tool becamecloser. The difference between this final pressure and the previouslyrecorded pressure is referred to as the slot pressure drop. The slotpressure drop in this example was 4.13×10⁶ Pa (600 psi) providingsufficient pressure to drive the molten polymer into the structuredcavities formed by the tool negative. The film thereby formed andstructured, was conveyed by the tool rotation from the slot, quenchedwith additional air cooling, stripped from the tool and wound into aroll. Including the height of the structures, the total thickness of thecast film (T) was about 600 microns.

The cast and wound polymer film closely replicated the tool structure.Using contact profilometry, (e.g. a KLA-Tencor P-10 with a 60° 2 micronradius stylus). A clear, reasonably sharp prismatic structure wasidentified on the surface of the film. The measured profile exhibitedthe expected, nearly right triangular form with straight edges and aslightly rounded apex. The replicated prisms on the polymeric filmsurface were measured to have a basal width (BW) of microns and a height(P) of 23.5 microns. The peak-to-peak spacing (PS) was approximately thesame as the basal width (BW). The profilometry is limited to about amicron in resolution due to the shape and size of the stylus probe andthe actual apex may be considerably higher. The tool is also imperfectand small deviations from nominal sizing can exist. To bettercharacterize the actual extent of fill, e.g. characterize the precisionof replication with the tool, the profilometry cross-section was fit toa triangle. Using data from the measured profile, the edges were fit asstraight lines along the legs of the cross-section between 5 and 15micron height as measured from the base. An ideal apex height of 24.6microns with an included apex angle of 91.10 was calculated. A ratio ofthe profile-measured cross-sectional area to the ideal calculatedcross-sectional area provided a calculated fill of 98.0%.

The structured cast film was stretched in a nearly truly uniaxial manneralong the continuous length direction of the prisms using the batchtenter process. The film was preheated to nominally 158° C. forstretched at this temperature over 90 seconds at a uniform speed (edgeseparation) to a final stretch ratio of about 6. The structured surfacesmaintained a prismatic shape with reasonably straight cross-sectionaledges (reasonably flat facets) and approximately similar shape.

The same contact profilometry as used on the cast film was used tomeasure the stretched film. The basal width after stretch (BW′) wasmeasured by microscopy cross-sectioning to be 22 microns and the peakheight after stretch (P′) was measured to be 8.5 microns. The finalthickness of the film (T′), including the structured height, wascalculated to be about 220 microns. The indices of refraction weremeasured on the backside of the stretched film using a Metricon PrismCoupler as available from Metricon, Piscataway, N.J., at a wavelength of632.8 nm. The indices along the first in-plane (along the prisms),second in-plane (across the prisms) and in the thickness direction weremeasured to be 1.790, 1.577 and 1.554 respectively. The relativebirefringence in the cross-sectional plane of this stretched materialwas thus 0.10.

Using the profilometry data, the ratio of the apparent cross-sectionalareas provide a measured estimate of the stretch ratio of 6.4,uncorrected for density changes upon stretching and orientation. Usingthis value of 6.4 for the stretch ratio and the profilometry data, theshape retention parameter was calculated to be 0.94.

Example 5

A co-polymer (so-called 40/60 coPEN) comprising 40 mol % polyethyleneterephthalate (PET) and 60 mol % polyethylene naphthalate character, asdetermined by the carboxylate (terephthalate and naphthalate) moiety(sub-unit) ratios, was made in a reactor vessel. The inherent viscosity(I.V.) was about 0.5.

The 40/60 coPEN resin pellets were dried to remove residual water andloaded into the extrusion hopper under a nitrogen purge. The 40/60 coPENwas extruded with a decreasing temperature profile of 285° C. to 277° C.within the extruder and the continuing melt train through to the die setat 288° C. Melt train pressures were continuously monitored and anaverage taken at the final monitored position along the melt train priorto bringing the die into close proximity to the tool onto which thepolymer film is formed simultaneously with the structuring of a firstsurface of that film against the tool.

The tool was a structured belt having the desired negative version ofthe structured surface formed on the cast film. The structured surfacecomprised a repeating and continuous series of isosceles righttriangular prisms, with basal widths (BW) of 50 microns and height (P)of nearly 25 microns. The basal vertices of the individual prisms wereshared by their adjoining, neighboring structures. The prisms werealigned along the casting (MD) direction. The structured surface of thetool was coated with a fluorochemical benzotriazole having the formula

where R_(f) is C₄F₉ and R is —(CH₂)₆—, as disclosed in U.S. Pat. No.6,376,065. The tool was mounted on a temperature-controlled rotating canwhich provides a continuous motion of the tool surface along the casting(MD) direction. The measured surface temperature of the tool averaged102° C.

The die orifice through which the molten polymer exits the melt trainwas brought into close proximity with the rotating belt tool forming afinal slot between the tool and die. The pressure at the final monitoredposition along the melt train increased as the die and tool becamecloser. The difference between this final pressure and the previouslyrecorded pressure is referred to as the slot pressure drop. The slotpressure drop in this example was 4.23×10⁶ Pa (614 psi) providingsufficient pressure to drive the molten polymer into the structuredcavities formed by the tool negative. The film thereby formed andstructured, was conveyed by the tool rotation from the slot, quenchedwith additional air cooling, stripped from the tool and wound into aroll. Including the height of the structures, the total thickness of thecast film (T) was about 560 microns.

The cast and wound polymer film closely replicated the tool structure.Using contact profilometry, (e.g. a KLA-Tencor P-10 with a 60° 2 micronradius stylus), a clear, reasonably sharp prismatic structure wasidentified on the surface of the film. The measured profile exhibitedthe expected, nearly right triangular form with straight edges and aslightly rounded apex. The replicated prisms on the polymeric filmsurface were measured to have a basal width (BW) of 49.9 microns and aheight (P) of 23.5 microns. The peak-to-peak spacing (PS) wasapproximately the same as the basal width (BW). The profilometry islimited to about a micron in resolution due to the shape and size of thestylus probe and the actual apex may be considerably higher. The tool isalso imperfect and small deviations from nominal sizing can exist. Tobetter characterize the actual extent of fill, e.g. characterize theprecision of replication with the tool, the profilometry cross-sectionwas fit to a triangle. Using data from the measured profile, the edgeswere fit as straight lines along the legs of the cross-section between 5and 15 micron height as measured from the base. An ideal apex height of24.6 microns with an included apex angle of 91.1 was calculated. A ratioof the profile-measured cross-sectional area to the ideal calculatedcross-sectional area provided a calculated fill of 98.0%.

The structured cast film was stretched in a nearly truly uniaxial manneralong the continuous length direction of the prisms. Using a laboratorystretcher. The film was preheated to 103° C. for 60 seconds andstretched at this temperature over 20 seconds at a uniform speed (edgeseparation) to a final stretch ratio of about 6. The structured surfacesmaintained a prismatic shape with reasonably straight cross-sectionaledges (reasonably flat facets) and approximately similar shape. Theindices of refraction were measured on the backside of the stretchedfilm using a Metricon Prism Coupler as available from Metricon,Piscataway, N.J., at a wavelength of 632.8 nm. The indices along thefirst in-plane (along the prisms), second in-plane (across the prisms)and in the thickness direction were measured to be 1.758, 1.553 and1.551 respectively. The relative birefringence in the cross-sectionalplane of this stretched material was thus 0.0097.

Example 6

A multilayer optical film made according to the procedures as describedin examples 1-4 of U.S. Patent Application Publication 2004/0227994 A1was cast and the protective polypropylene skin layer removed. The lowindex polymer used was a co-PET.

The multilayer optical film was cut into a sheet and dried in an oven at60° C. for a minimum of 2 hours. The platens were heated to 115° C. Thefilm was stacked in a construction of layers in the order: cardboardsheet, chrome plated brass plates (approx 3 mm thick), release liner,nickel microstructure tool, multilayer optical film, release liner,chrome plated brass plate (approx 3 mm thick), and cardboard sheet. Theconstruction was placed between the platens and closed. A pressure of1.38×10⁵ Pa (20 psi) was maintained for 60 seconds.

The structured surface of the nickel microstructure tool comprised arepeating and continuous series of triangular prisms, with a 90° apexangle, basal widths (BW) of 10 microns and a height (P) of about 5microns. The basal vertices of the individual prisms were shared bytheir adjoining, neighboring structures.

The embossed sheets were cut to an aspect ratio of 10:7 (along to acrossthe grooves). The structured multilayer optical film was stretched in anearly truly uniaxial manner along the continuous length direction ofthe prisms using a batch tenter process. The film was preheated tonearly 100° C., stretched to a stretch ratio around 6 over about 20seconds, and then the stretching was reduced by about 10% while still inthe tenter at stretch temperature, to control shrinkage in the film. Thefinal thickness of the film (T′), including the structured height, wasmeasured to be 150 microns. The indices of refraction were measured onthe backside of the stretched film using a Metricon Prism Coupler asavailable from Metricon, Piscataway, N.J., at a wavelength of 632.8 nm.The indices along the first in-plane (along the prisms), second in-plane(across the prisms) and in the thickness direction were measured to be1.699, 1.537 and 1.534 respectively. The birefringence in thecross-sectional plane of this stretched material was thus 0.018.

Example 7

An oriented, microreplicated structure was constructed as follows: 90°prismatic grooves at 125 micron pitch were embossed into an 0.010 inchthick film of cast PEN(polyether naphalate) by compression molding at125 C for 4 minutes. The tool structured film was quenched in anicewater. After removal and drying of the film, the film was thenuniaxially stretched 5× along the long axis of the grooves at 128 C.This resulted in transverse shrinkage of 5%, yielding a final pitch ofapproximately 62 microns. The refractive index was measured to be 1.84along the oriented axis and 1.53 in the transverse direction. Theindices of refraction were measured on the flat backside of the filmusing a Metricon Prism Coupler at a wavelength of 632.8 nm.

A piece of the oriented microstructured film was subsequently adhered toa glass microscope slide with the structured surface facing the slideusing a UV curable acrylate resin with an isotropic refractive index1.593. The acrylate resin was cured by multiple passes through the UVchamber—3 times on each side to ensure full cure of the resin.

A Helium-Neon laser beam was passed through the slide mounted orientedstructured film. The HeNe laser was cleaned to a uniform linearpolarization by passing through a Glan-Thompson polarizer. Theordinary-ray (o-ray) passed through the structure with only a smalldegree of splitting, where the half angle of the zeroth order divergencewas found to be approximately 2°. A half-wave plate was then insertedimmediately after the Glan-Thompson in order to rotate the laser beam90° to the orthogonal polarization (e-ray). The zeroth order beam showeda divergence half angle of approximately 8°, or 4× the divergence of theo-ray.

Example 8

A filter according to the invention was made from a polymer film thatwas melt-cast onto a micro-grooved tool, length oriented, fibrillated,and electret charged. The film was cast and length oriented in acontinuous process, depicted generally in FIG. 17, employing standardextrusion techniques using a 400 melt index polypropylene homopolymerresin, type 3505 from Exxon Co (now ExxonMobil Chemical Co., Houston,Tex.). Molten resin at a temperature of approximately 188° C. wasdelivered from an extruder 50 to the nip formed between a micro-groovedpattern roll or tool 52 and a rubber nip roll 54. Temperature of themicro-grooved tool 52 was maintained at approximately 52° C. with thecooperating rubber nip roll 54 maintained at approximately 27° C. Thenip force between the tool 52 and rubber nip roll 54 was maintained at29 N/cm. The film path continued past heated rolls 55 and 56, draw zone57 and rubber nip roll 58 and associated drive roll 59. The draw ratio,amount of elongation of the film based on the ratio of the surfacespeeds of the tool and the final winding roll (drive roll 59), was fourto one. The micro-grooved tool 52 had continuous, circumferentiallyaligned, “V” shaped grooves disposed on its surface, with features likethat shown in FIG. 18. The grooves on the tool abutted one anotheracross the face of the tool and had a depth of 165 μm with peak-to-peakspacing of 120 μm and an angle forming the groove of 40°. The filmproduced had a basis weight of 22.1 g/m² and a total thickness of 81 μm.Features on the surface of the film were approximately 50 μm in height.FIG. 19 is an illustrative photo of a sample of the stretched film madeas described above.

The film formed in the first extrusion step was fibrillated using ahydroentangler 60 (FIG. 20), in this case exemplified by Hydrolace 350System, available from CEL International LTD., Coventry, England. Ahydroentangler is a device that uses high-pressure water jets to impingeon and entangle fibrous webs. In this application the hydroentangler wasemployed to separate features of the film into individual strands orbundles of strands. Prior to fibrillation, two lengths of film werelayered, one placed on top of one another, and passed through thehydroentangler with their microstructure features aligned in the machinedirection of the hydroentangler. The two film layers were fixed togetherat their ends with adhesive tape and passed under a succession of sixjet-strips 62, 63, 64, 65, 66 and 67 that were directed towards, and2.54 cm above, perforated conveyor drums 68 and 70 as is indicated inFIG. 20. The conveyor speed of the hydroentangler 60 was maintained at arate of 5 m/min. Each jet-strip had a line of evenly spaced jet holes,15.75 holes/cm, with each hole having a diameter of 120 microns. Thefirst jet-strip 62 was operated at a water pressure of 5 mega Pascal(MPa), while the second through sixth jet strips, indicated as jetstrips 63, 64, 65, 66 and 67, were operated at 7.5 MPa, 10 MPa, 5 MPa,7.5 MPa, and 10 Mpa, respectively. After passing through thehydroentangler the film segments were allowed to air dry prior toelectret charging.

With the ends of the fibrillated film layers fixed with adhesive tapethe film was electret charged using a DC corona discharge device. Thefilm was electret charged by exposure to a high voltage field in amethod generally described in U.S. Pat. No. 3,998,916 (van Turnhout).The film was placed in contact with an aluminum ground plane, and thenpassed twice under an electrically positive 25 kV DC corona source, inair, at a rate of about 1.77 meters/min, while maintaining a current toground plane of about 0.017 mA/cm of corona source length. The distancefrom corona source to ground was about 4 cm. The electret charged filmwas then formed into a filter bed of fibrils and tested for filterperformance.

The charged film was formed into a bed of fibrils by cutting theelectret charged fibrillated film into five to six centimeter longsegments and mixing by hand to form a 2.67 gram mass of fibrils. Thefibril mass was then formed by hand into the shape of a cylindricalfilter bed, 11.4 cm in diameter and 0.6 cm deep. This fiber bed wasplaced in a test fixture and exposed to a particle challenge andevaluated for particle capture efficiency and pressure drop as outlinedin the National Institute for Occupational Safety and Health (NIOSH)standard and described in United States Code of Federal Regulations,42CFR84. Using a model 8127 automatic filter tester from TSIIncorporated, St. Paul Minn., an initial particle penetration wasdetermined by forcing 0.3 micrometer diameter dioctyl phthalate (DOP)particles at a concentration of 103 mg/m³ (generated using a TSI No. 212sprayer with four orifices and 207 kPa aerosol pressure, 50 L/mindilution air, and neutralizer on) through the filter bed at a rate of42.5 L/min (a face velocity of 6.9 centimeters per second). The samplewas exposed to the DOP aerosol for 30 seconds until the readingsstabilized. Particle penetration was measured at 47% with a pressuredrop of just 1.95 Pa (0.2 mmH₂O). DOP penetration and pressure drop wereused to calculate a filter quality factor, Q, from the natural log (ln)of the DOP penetration using the following formula:

${Q\left\lbrack {{1/{mm}}\mspace{14mu} H_{2}O} \right\rbrack} = \frac{{- {Ln}}\frac{{DOPPenetration}(\%)}{100}}{{PressureDrop}\left\lbrack {{mm}\mspace{14mu} H_{2}O} \right\rbrack}$

The quality factor for the web tested was 3.8; quality factors greaterthan 2 are generally accepted to represent efficient filters.

Example 9

A film according to the invention was produced. The film comprised aphase-separated film that had been embossed with linear features andthen biaxially stretched to both elongate the feature and cause the filmto become microporous. A microporous precursor (unstretched) film,formed in the manner generally described in U.S. Pat. No. 4,539,256(Shipman), was made using a melt-blended composition of homopolymerpolypropylene and mineral oil. The unstretched phase-separated film wasproduced from a 46:54 blend of mineral oil (80 centistokes viscosity)and polypropylene (0.8 melt flow index). Two layers of the film wereembossed with a micro-grooved tool in a hydraulic press heated to 155°C. at a pressure of 574 kPa for 30 seconds. The tool had linear groovesthat abutted each other running along the length of one surface of a12.7 cm×30.5 cm plate. The grooves on the master tool had a depth of92.9 μm with peak-to-peak spacing of 67.8 μm and an angle forming thegroove of 45′. The groove was tilted 17.50 from normal. The embossedfilm was then placed in a small tentering frame and stretched to 1.5times its width (perpendicular to the features) and two times its width(parallel to the features). The resulting microstructured-microporousfilm was observed microscopically to have microporous structurethroughout the film including the features, as is evident in FIG. 21.Films of this type might find use in filtration, fuel cell, or apparelapplications where having an integral feature in the microporous filmwould be beneficial.

Example 10 Crystallinity Index Method

Crystallinity index was determined using transmission geometry datacollected in the form of survey scans through use of a Bruker GADDSMicrodiffractometer (available from Bruker AXS Inc. of Madison, Wis.),CuK_(α) radiation source, and HiStar 2D position sensitive detectorregistry of the scattered radiation. Samples were positioned so as toplace the lengthwise dimension in the vertical plane of thediffractometer. The diffractometer was fitted with pinhole collimationthat used a 300 micron aperture and graphite incident beammonochromator. The detector was centered at 0 degrees (2θ), and nosample tilt was employed. Data were accumulated for 15 minutes at asample to detector distance of 6 cm. X-ray generator settings of 50 kVand 100 mA were employed; values of crystallinity were reported as anindex of the percent crystallinity. Two-dimensional data were radiallysummed to produce a conventional 1D diffraction pattern. The resultingpattern was subjected to profile fitting using the program ORIGIN(Origin Lab Co., Northhampton, Mass.) to separate amorphous andcrystalline polymer scattering components. For profile fitting, aparabolic background model and a Gaussian peak shape model wereemployed. Crystallinity index was evaluated as the ratio of crystallinescattering above background to total amorphous and crystallinescattering above background within the 10 to 35 degrees (2θ) scatteringangle range.

Example 10b

A microstructured and drawn film of the invention was made from apolymer that was melt-cast onto a micro-grooved master tool and lengthoriented. The microreplicated film was produced by extruding polymerinto a nip, with one surface being formed from the metal tool and theother surface being formed from a flat silicone rubber belt surface. Thefilm was cast in a continuous process employing standard extrusiontechniques using a 9.0 melt index polypropylene homopolymer resin, type3576X from Total Petrochemicals USA, Houston, Tex. Molten resin at atemperature of approximately 229° C. was delivered to the nip formedbetween a micro-grooved master tool and a rubber belt. The temperatureof the micro-grooved casting roll was maintained at approximately 60° C.with the cooperating rubber belt maintained with a surface temperatureof 171-177° C. The nip force between the casting roll and rubber beltwas maintained at 80.8 N/cm. The micro-grooved master tool hadcontinuous, circumferentially aligned, “V” shaped grooves disposed onits surface, with features like that shown in FIG. 22. The grooves onthe master tool had larger features having a depth of 66 μm withpeak-to-peak spacing of 236 μm and an angle forming the groove of 54°.The smaller features had a depth of 39 μm with peak-to-peak spacing of52 μm and an angle forming the groove of 54°. The film produced had abasis weight of 60.7 g/m² and a total thickness of 106 μm. Theseparation span between adjacent features was 13 μm. Features on thesurface of the film, shown in FIG. 23, followed the form of the mastertool and had a height of 49 μm for the larger features and 30 μm for thesmall features with a land thickness of 57 um.

A film sample was uniaxially drawn, using a Karo IV Laboratory drawingmachine, available from Bruickner ServTec, Siegsdorf, Germany. The filmsample was cut into a 100 mm×100 mm square, and drawn along the axis ofthe microstructure features. After equilibrating to a temperature of150° C. the film sample was draw at a rate of 3.5 m/min to length fivetimes their original length resulting in draw ratio of 5:1. The samplewas then subjected to x-ray crystallography analysis to determine thedegree of crystallinity, result are given in Table 1.

Comparative Example 10b

A drawn and then microstructured film was produced and tested asdescribed in Example 1b except that the microstructured features wereapplied to the film after it had been drawn. Using the same material asdescribed in Example 10b, the film of the comparative was produced byextruding a thin molten sheet between a smooth metal casting roll and asmooth rubber roll. Molten resin at a temperature of approximately 294°C. was delivered to the nip formed between the smooth metal roll, andsmooth rubber roll. The metal casting roll was maintained at atemperature of 22° C., while the cooperating rubber roll was cooledusing 14° C. water. The surface temperature of the rubber roll was notmeasured but was appreciable higher than the temperature of the coolingwater. The nip force between the casting roll and rubber belt wasmaintained at 169 N/cm. The resultant film had a nominal thickness of 70μm and a basis weight of 59 g/m². A sample of the film was taken anddrawn as described in Example 10b. The drawn sample was thenmicro-machined on one surface using a diamond-tipped tool. Features ofthe same general configuration and orientation to the draw direction, asthe drawn film produced in Example 10b, were cut into the film. Tomachine the film, a section was mounted on a precision turning drum, thedrum was rotated at a surface speed of 31.7 m/min and the diamond tipbrought to contact the film. The 90° sharp-tip diamond tool was plungedinto the film to produce abutting channels that were 12-15 um deep. AModel SS-156 tooling machine, from Pneumo Precision Products Inc. wasused to machine the film (Pneumo Precision Products Inc. became thePneuma ultra-precision machine tool division of Taylor Hobson (formerlyRank Taylor Hobson/Rank Pneumo), and has since merged into Precitech,Inc., Keene, N.H.). The sample was then subjected to x-raycrystallography analysis to determine the degree of crystallinity,result are given in Table 1.

TABLE 1 Example 10b C10b Crystallinity Index 0.35 0.29

Results: As is illustrated by the crystallinity index the inventive filmretained a desirably greater crystalline level than the comparativefilm, even though both films were drawn to an equivalent degree and hadsimilar microstructure features. The results indicate that thecrystalline morphology of the microstructure features is retained forthe inventive film but lost in the comparative film, where themicrostructure features are formed into the film after drawing. Higherdegree of crystallinity results in desirable mechanical and opticalproperties.

The invention claimed is:
 1. A method of forming layered arrays of fiberelements comprising: forming a polymeric film having a body having afirst surface and a second surface and having a longitudinal dimension,and a plurality of elongate microstructured surface features disposed onthe first surface of the body in a direction substantially parallel tothe longitudinal dimension of the body, wherein the microstructuredsurface features are substantially parallel; stretching the polymericfilm in a direction substantially parallel to the longitudinal dimensionof the body; separating the stretched polymeric film along generallylongitudinally disposed separation lines to define a plurality ofdiscrete fiber elements, wherein one or more of the fiber elements haveat least one microstructured surface feature thereon; arranging a firstlayer of the discrete fibers elements in an array of fiber elements sothat the longitudinal dimensions of the fiber elements of the firstlayer are aligned in a first direction wherein the longitudinaldimensions of the fiber elements of the first layer are substantiallyparallel; and overlaying the first layer with a second layer of thediscrete fiber elements defined by another array of fiber elements sothat the longitudinal dimensions of the fiber elements of the secondlayer are aligned in a second direction wherein the longitudinaldimensions of the fiber elements of the second layer are substantiallyparallel.
 2. The method of claim 1 wherein at least some of themicrostructured surface features on the fiber elements of the firstlayer engage at least some of the microstructured surface features onthe fiber elements of the second layer.
 3. The method of claim 1 whereinthe first and second directions are different.
 4. The method of claim 1wherein the first and second directions are substantially parallel. 5.The method of claim 1, wherein the forming step further comprises:defining at least one generally laterally disposed lateral separationline across the longitudinal dimension of the body; and separating eachfiber into one or more fiber segments.
 6. The method of claim 1 whereinthe film separating and fiber separating steps occur simultaneously. 7.The method of claim 1, further comprising: exposing the polymeric filmto an electric field.
 8. The method of claim 7 wherein the exposing stepdefines the film as an electret.
 9. The method of claim 8, wherein theforming step further comprises: defining the shape of at least one ofthe microstructure features to enhance electrical field effects.
 10. Themethod of claim 8 wherein the exposing step precedes the stretchingstep.
 11. The method of claim 8 wherein the stretching step precedes theexposing step.
 12. The method of claim 7, wherein the body has a firstthickness and the microstructure features have a second thickness, andwherein after stretching, the ratio of the first thickness to the secondthickness is at most
 2. 13. The method of claim 7 wherein the formingstep further comprises: defining the longitudinal separation lines onthe polymeric film.
 14. The method of claim 7, and further comprising:laterally cutting the fiber elements; mixing the cut fiber elements; andforming a filtration layer of mixed fiber elements.
 15. The method ofclaim 7 wherein at least one microstructure feature is continuous alongthe first surface of the body.
 16. The method of claim 7 wherein atleast one microstructure feature is discontinuous along the firstsurface of the body.
 17. The method of claim 7 wherein at least twoadjacent microstructure features are spaced apart laterally by aseparation span of the body.
 18. The method of claim 17 wherein at leastone of the separation lines extends within one of the separation spansand is spaced laterally from the microstructure features on each side ofthat separation span.
 19. The method of claim 7 wherein at least one ofthe fiber elements has a plurality of microstructure features thereon.20. The method of claim 7 wherein at least one microstructure featurehas at least one discontinuity.